[PATCH v2 00/45] CPU hotplug: stop_machine()-free CPU hotplug, part 1

[PATCH v2 00/45] CPU hotplug: stop_machine()-free CPU hotplug, part 1

[Srivatsa S. Bhat]

Hi,

This patchset is a first step towards removing stop_machine() from the
CPU hotplug offline path. It introduces a set of APIs (as a replacement to
preempt_disable()/preempt_enable()) to synchronize with CPU hotplug from
atomic contexts.

The motivation behind getting rid of stop_machine() is to avoid its
ill-effects, such as performance penalties[1] and the real-time latencies it
inflicts on the system (and also things involving stop_machine() have often
been notoriously hard to debug). And in general, getting rid of stop_machine()
from CPU hotplug also greatly enhances the elegance of the CPU hotplug design
itself.

Getting rid of stop_machine() involves building the corresponding
infrastructure in the core CPU hotplug code and converting all places which
used to depend on the semantics of stop_machine() to synchronize with CPU
hotplug.

This patchset builds a first-level base infrastructure on which tree-wide
conversions can be built upon, and also includes the conversions themselves.
We certainly need a few more careful tree-sweeps to complete the conversion,
but the goal of this patchset is to introduce the core pieces and to get the
first batch of conversions in, while covering a reasonable bulk among them.

This patchset also has a debug infrastructure to help with the conversions -
with the newly introduced CONFIG_DEBUG_HOTPLUG_CPU option turned on, it
prints warnings whenever the need for a conversion is detected. Patches 4-7
build this framework. Needless to say, I'd really appreciate if people could
test kernels with this option turned on and report omissions or better yet,
send patches to contribute to this effort.

[It is to be noted that this patchset doesn't replace stop_machine() yet,
so the immediate risk in having an unconverted (or converted) call-site
is nil, since there is no major functional change involved.]

Once the conversion gets completed, we can finalize on the design of the
stop_machine() replacement and use that in the core CPU hotplug code. We have
had some discussions in the past where we debated several different
designs[2]. We'll revisit that with more ideas once this conversion gets over.


This patchset applies on current tip:master. It is also available in the
following git branch:

git://github.com/srivatsabhat/linux.git  stop-mch-free-cpuhp-part1-v2


Thank you very much!


Changes in v2:
-------------
* Build-fix for !HOTPLUG_CPU case.
* Minor code cleanups, and added a few comments.


References:
----------

1. Performance difference between CPU Hotplug with and without
   stop_machine():
   http://article.gmane.org/gmane.linux.kernel/1435249

2. Links to discussions around alternative synchronization schemes to
   replace stop_machine() in the CPU Hotplug code:

   v6: http://lwn.net/Articles/538819/
   v5: http://lwn.net/Articles/533553/
   v4: https://lkml.org/lkml/2012/12/11/209
   v3: https://lkml.org/lkml/2012/12/7/287
   v2: https://lkml.org/lkml/2012/12/5/322
   v1: https://lkml.org/lkml/2012/12/4/88

3. Links to previous versions of this patchset:
   v1: http://lwn.net/Articles/556138/

--
 Srivatsa S. Bhat (45):
      CPU hotplug: Provide APIs to prevent CPU offline from atomic context
      CPU hotplug: Clarify the usage of different synchronization APIs
      Documentation, CPU hotplug: Recommend usage of get/put_online_cpus_atomic()
      CPU hotplug: Add infrastructure to check lacking hotplug synchronization
      CPU hotplug: Protect set_cpu_online() to avoid false-positives
      CPU hotplug: Sprinkle debugging checks to catch locking bugs
      CPU hotplug: Expose the new debug config option
      CPU hotplug: Convert preprocessor macros to static inline functions
      smp: Use get/put_online_cpus_atomic() to prevent CPU offline
      sched/core: Use get/put_online_cpus_atomic() to prevent CPU offline
      migration: Use raw_spin_lock/unlock since interrupts are already disabled
      sched/fair: Use get/put_online_cpus_atomic() to prevent CPU offline
      timer: Use get/put_online_cpus_atomic() to prevent CPU offline
      sched/rt: Use get/put_online_cpus_atomic() to prevent CPU offline
      rcu: Use get/put_online_cpus_atomic() to prevent CPU offline
      tick-broadcast: Use get/put_online_cpus_atomic() to prevent CPU offline
      time/clocksource: Use get/put_online_cpus_atomic() to prevent CPU offline
      softirq: Use get/put_online_cpus_atomic() to prevent CPU offline
      irq: Use get/put_online_cpus_atomic() to prevent CPU offline
      net: Use get/put_online_cpus_atomic() to prevent CPU offline
      block: Use get/put_online_cpus_atomic() to prevent CPU offline
      percpu_counter: Use get/put_online_cpus_atomic() to prevent CPU offline
      infiniband: ehca: Use get/put_online_cpus_atomic() to prevent CPU offline
      [SCSI] fcoe: Use get/put_online_cpus_atomic() to prevent CPU offline
      staging/octeon: Use get/put_online_cpus_atomic() to prevent CPU offline
      x86: Use get/put_online_cpus_atomic() to prevent CPU offline
      perf/x86: Use get/put_online_cpus_atomic() to prevent CPU offline
      KVM: Use get/put_online_cpus_atomic() to prevent CPU offline
      kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline
      x86/xen: Use get/put_online_cpus_atomic() to prevent CPU offline
      alpha/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
      blackfin/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
      cris/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
      hexagon/smp: Use get/put_online_cpus_atomic() to prevent CPU offline
      ia64: irq, perfmon: Use get/put_online_cpus_atomic() to prevent CPU offline
      ia64: smp, tlb: Use get/put_online_cpus_atomic() to prevent CPU offline
      m32r: Use get/put_online_cpus_atomic() to prevent CPU offline
      MIPS: Use get/put_online_cpus_atomic() to prevent CPU offline
      mn10300: Use get/put_online_cpus_atomic() to prevent CPU offline
      powerpc, irq: Use GFP_ATOMIC allocations in atomic context
      powerpc: Use get/put_online_cpus_atomic() to prevent CPU offline
      powerpc: Use get/put_online_cpus_atomic() to avoid false-positive warning
      sh: Use get/put_online_cpus_atomic() to prevent CPU offline
      sparc: Use get/put_online_cpus_atomic() to prevent CPU offline
      tile: Use get/put_online_cpus_atomic() to prevent CPU offline

 Documentation/cpu-hotplug.txt                 |   20 +++-
 arch/alpha/kernel/smp.c                       |   19 ++-
 arch/blackfin/mach-common/smp.c               |    4 -
 arch/cris/arch-v32/kernel/smp.c               |    5 +
 arch/hexagon/kernel/smp.c                     |    3 +
 arch/ia64/kernel/irq_ia64.c                   |   15 +++
 arch/ia64/kernel/perfmon.c                    |    8 +
 arch/ia64/kernel/smp.c                        |   12 +-
 arch/ia64/mm/tlb.c                            |    4 -
 arch/m32r/kernel/smp.c                        |   16 +--
 arch/mips/kernel/cevt-smtc.c                  |    7 +
 arch/mips/kernel/smp.c                        |   16 +--
 arch/mips/kernel/smtc.c                       |   12 ++
 arch/mips/mm/c-octeon.c                       |    4 -
 arch/mn10300/mm/cache-smp.c                   |    3 +
 arch/mn10300/mm/tlb-smp.c                     |   17 ++-
 arch/powerpc/kernel/irq.c                     |    9 +-
 arch/powerpc/kernel/machine_kexec_64.c        |    4 -
 arch/powerpc/kernel/smp.c                     |    4 +
 arch/powerpc/kvm/book3s_hv.c                  |    5 +
 arch/powerpc/mm/mmu_context_nohash.c          |    3 +
 arch/powerpc/oprofile/cell/spu_profiler.c     |    3 +
 arch/powerpc/oprofile/cell/spu_task_sync.c    |    4 +
 arch/powerpc/oprofile/op_model_cell.c         |    6 +
 arch/sh/kernel/smp.c                          |   12 +-
 arch/sparc/kernel/smp_64.c                    |   12 +-
 arch/tile/kernel/module.c                     |    3 +
 arch/tile/kernel/tlb.c                        |   15 +++
 arch/tile/mm/homecache.c                      |    3 +
 arch/x86/kernel/apic/io_apic.c                |   21 +++-
 arch/x86/kernel/cpu/mcheck/therm_throt.c      |    4 -
 arch/x86/kernel/cpu/perf_event_intel_uncore.c |    6 +
 arch/x86/kvm/vmx.c                            |   13 +-
 arch/x86/mm/tlb.c                             |   14 +--
 arch/x86/xen/mmu.c                            |    9 +-
 block/blk-softirq.c                           |    3 +
 drivers/infiniband/hw/ehca/ehca_irq.c         |    5 +
 drivers/scsi/fcoe/fcoe.c                      |    7 +
 drivers/staging/octeon/ethernet-rx.c          |    3 +
 include/linux/cpu.h                           |   29 +++++
 include/linux/cpumask.h                       |   59 ++++++++++-
 kernel/cpu.c                                  |  138 +++++++++++++++++++++++++
 kernel/irq/manage.c                           |    7 +
 kernel/irq/proc.c                             |    3 +
 kernel/rcutree.c                              |    4 +
 kernel/sched/core.c                           |   27 ++++-
 kernel/sched/fair.c                           |   14 ++-
 kernel/sched/rt.c                             |   14 +++
 kernel/smp.c                                  |   52 +++++----
 kernel/softirq.c                              |    3 +
 kernel/time/clocksource.c                     |    5 +
 kernel/time/tick-broadcast.c                  |    8 +
 kernel/timer.c                                |    4 +
 lib/Kconfig.debug                             |    8 +
 lib/cpumask.c                                 |    8 +
 lib/percpu_counter.c                          |   10 ++
 net/core/dev.c                                |    9 +-
 virt/kvm/kvm_main.c                           |    8 +
 58 files changed, 614 insertions(+), 129 deletions(-)


Regards,
Srivatsa S. Bhat
IBM Linux Technology Center

[PATCH v2 01/45] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

[Srivatsa S. Bhat]

The current CPU offline code uses stop_machine() internally. And disabling
preemption prevents stop_machine() from taking effect, thus also preventing
CPUs from going offline, as a side effect.

There are places where this side-effect of preempt_disable() (or equivalent)
is used to synchronize with CPU hotplug. Typically these are in atomic
sections of code, where they can't make use of get/put_online_cpus(), because
the latter set of APIs can sleep.

Going forward, we want to get rid of stop_machine() from the CPU hotplug
offline path. And then, with stop_machine() gone, disabling preemption will
no longer prevent CPUs from going offline.

So provide a set of APIs for such atomic hotplug readers, to prevent (any)
CPUs from going offline. For now, they will default to preempt_disable()
and preempt_enable() itself, but this will help us do the tree-wide conversion,
as a preparatory step to remove stop_machine() from CPU hotplug.

(Besides, it is good documentation as well, since it clearly marks places
where we synchronize with CPU hotplug, instead of combining it subtly with
disabling preemption).

In future, when actually removing stop_machine(), we will alter the
implementation of these APIs to a suitable synchronization scheme.

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpu.h |   20 ++++++++++++++++++++
 kernel/cpu.c        |   38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 9f3c7e8..a57b25a 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -17,6 +17,8 @@
 #include <linux/node.h>
 #include <linux/compiler.h>
 #include <linux/cpumask.h>
+#include <linux/percpu.h>
+#include <linux/smp.h>
 
 struct device;
 
@@ -175,6 +177,8 @@ extern struct bus_type cpu_subsys;
 
 extern void get_online_cpus(void);
 extern void put_online_cpus(void);
+extern unsigned int get_online_cpus_atomic(void);
+extern void put_online_cpus_atomic(void);
 extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
@@ -202,6 +206,22 @@ static inline void cpu_hotplug_driver_unlock(void)
 #define put_online_cpus()	do { } while (0)
 #define cpu_hotplug_disable()	do { } while (0)
 #define cpu_hotplug_enable()	do { } while (0)
+
+static inline unsigned int get_online_cpus_atomic(void)
+{
+	/*
+	 * Disable preemption to avoid getting complaints from the
+	 * debug_smp_processor_id() code.
+	 */
+	preempt_disable();
+	return smp_processor_id();
+}
+
+static inline void put_online_cpus_atomic(void)
+{
+	preempt_enable();
+}
+
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 198a388..2d03398 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -154,6 +154,44 @@ void cpu_hotplug_enable(void)
 	cpu_maps_update_done();
 }
 
+/*
+ * get_online_cpus_atomic - Prevent any CPU from going offline
+ *
+ * Atomic hotplug readers (tasks which wish to prevent CPUs from going
+ * offline during their critical section, but can't afford to sleep)
+ * can invoke this function to synchronize with CPU offline. This function
+ * can be called recursively, provided it is matched with an equal number
+ * of calls to put_online_cpus_atomic().
+ *
+ * Note: This does NOT prevent CPUs from coming online! It only prevents
+ * CPUs from going offline.
+ *
+ * Lock ordering rule: Strictly speaking, there is no lock ordering
+ * requirement here, but it is advisable to keep the locking consistent.
+ * As a simple rule-of-thumb, use these functions in the outer-most blocks
+ * of your critical sections, outside of other locks.
+ *
+ * Returns the current CPU number, with preemption disabled.
+ */
+unsigned int get_online_cpus_atomic(void)
+{
+	/*
+	 * The current CPU hotplug implementation uses stop_machine() in
+	 * the CPU offline path. And disabling preemption prevents
+	 * stop_machine() from taking effect. Thus, this prevents any CPU
+	 * from going offline.
+	 */
+	preempt_disable();
+	return smp_processor_id();
+}
+EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
+
+void put_online_cpus_atomic(void)
+{
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(put_online_cpus_atomic);
+
 #else /* #if CONFIG_HOTPLUG_CPU */
 static void cpu_hotplug_begin(void) {}
 static void cpu_hotplug_done(void) {}

[PATCH v2 02/45] CPU hotplug: Clarify the usage of different synchronization APIs

[Srivatsa S. Bhat]

We have quite a few APIs now which help synchronize with CPU hotplug.
Among them, get/put_online_cpus() is the oldest and the most well-known,
so no problems there. By extension, its easy to comprehend the new
set : get/put_online_cpus_atomic().

But there is yet another set, which might appear tempting to use:
cpu_hotplug_disable()/cpu_hotplug_enable(). Add comments to clarify
that this latter set is NOT for general use and must be used only in
specific cases where the requirement is really to _disable_ hotplug
and not just to synchronize with it.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2d03398..860f51a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -139,6 +139,13 @@ static void cpu_hotplug_done(void)
  * the 'cpu_hotplug_disabled' flag. The same lock is also acquired by the
  * hotplug path before performing hotplug operations. So acquiring that lock
  * guarantees mutual exclusion from any currently running hotplug operations.
+ *
+ * Note: In most cases, this is *NOT* the function you need. If you simply
+ * want to avoid racing with CPU hotplug operations, use get/put_online_cpus()
+ * or get/put_online_cpus_atomic(), depending on the situation.
+ *
+ * This set of functions is reserved for cases where you really wish to
+ * _disable_ CPU hotplug and not just synchronize with it.
  */
 void cpu_hotplug_disable(void)
 {

[PATCH v2 03/45] Documentation, CPU hotplug: Recommend usage of get/put_online_cpus_atomic()

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

So add documentation to recommend using the new get/put_online_cpus_atomic()
APIs to prevent CPUs from going offline, while invoking from atomic context.

Cc: Rob Landley <rob@landley.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 Documentation/cpu-hotplug.txt |   20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index 9f40135..7b3ca60 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -113,13 +113,18 @@ Never use anything other than cpumask_t to represent bitmap of CPUs.
 	#include <linux/cpu.h>
 	get_online_cpus() and put_online_cpus():
 
-The above calls are used to inhibit cpu hotplug operations. While the
+The above calls are used to inhibit cpu hotplug operations, when invoked from
+non-atomic contexts (because the above functions can sleep). While the
 cpu_hotplug.refcount is non zero, the cpu_online_mask will not change.
-If you merely need to avoid cpus going away, you could also use
-preempt_disable() and preempt_enable() for those sections.
-Just remember the critical section cannot call any
-function that can sleep or schedule this process away. The preempt_disable()
-will work as long as stop_machine_run() is used to take a cpu down.
+
+However, if you are executing in atomic context (ie., you can't afford to
+sleep), and you merely need to avoid cpus going offline, you can use
+get_online_cpus_atomic() and put_online_cpus_atomic() for those sections.
+Just remember the critical section cannot call any function that can sleep or
+schedule this process away. Using preempt_disable() will also work, as long
+as stop_machine() is used to take a CPU down. But we are going to get rid of
+stop_machine() in the CPU offline path soon, so it is strongly recommended
+to use the APIs mentioned above.
 
 CPU Hotplug - Frequently Asked Questions.
 
@@ -360,6 +365,9 @@ A: There are two ways.  If your code can be run in interrupt context, use
 		return err;
 	}
 
+   If my_func_on_cpu() itself cannot block, use get/put_online_cpus_atomic()
+   instead of get/put_online_cpus(), to prevent CPUs from going offline.
+
 Q: How do we determine how many CPUs are available for hotplug.
 A: There is no clear spec defined way from ACPI that can give us that
    information today. Based on some input from Natalie of Unisys,

[PATCH v2 04/45] CPU hotplug: Add infrastructure to check lacking hotplug synchronization

[Srivatsa S. Bhat]

Add a debugging infrastructure to warn if an atomic hotplug reader has not
invoked get_online_cpus_atomic() before traversing/accessing the
cpu_online_mask. Encapsulate these checks under a new debug config option
DEBUG_HOTPLUG_CPU.

This debugging infrastructure proves useful in the tree-wide conversion
of atomic hotplug readers from preempt_disable() to the new APIs, and
help us catch the places we missed, much before we actually get rid of
stop_machine(). We can perhaps remove the debugging checks later on.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Alex Shi <alex.shi@intel.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpumask.h |   12 ++++++
 kernel/cpu.c            |   89 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index d08e4d2..9197ca4 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -101,6 +101,18 @@ extern const struct cpumask *const cpu_active_mask;
 #define cpu_active(cpu)		((cpu) == 0)
 #endif
 
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU
+extern void check_hotplug_safe_cpumask(const struct cpumask *mask);
+extern void check_hotplug_safe_cpu(unsigned int cpu,
+				   const struct cpumask *mask);
+#else
+static inline void check_hotplug_safe_cpumask(const struct cpumask *mask) { }
+static inline void check_hotplug_safe_cpu(unsigned int cpu,
+					  const struct cpumask *mask)
+{
+}
+#endif
+
 /* verify cpu argument to cpumask_* operators */
 static inline unsigned int cpumask_check(unsigned int cpu)
 {
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 860f51a..5297ec1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -63,6 +63,92 @@ static struct {
 	.refcount = 0,
 };
 
+#ifdef CONFIG_DEBUG_HOTPLUG_CPU
+
+static DEFINE_PER_CPU(unsigned long, atomic_reader_refcnt);
+
+static int current_is_hotplug_safe(const struct cpumask *mask)
+{
+
+	/* If we are not dealing with cpu_online_mask, don't complain. */
+	if (mask != cpu_online_mask)
+		return 1;
+
+	/* If this is the task doing hotplug, don't complain. */
+	if (unlikely(current == cpu_hotplug.active_writer))
+		return 1;
+
+	/* If we are in early boot, don't complain. */
+	if (system_state != SYSTEM_RUNNING)
+		return 1;
+
+	/*
+	 * Check if the current task is in atomic context and it has
+	 * invoked get_online_cpus_atomic() to synchronize with
+	 * CPU Hotplug.
+	 */
+	if (preempt_count() || irqs_disabled())
+		return this_cpu_read(atomic_reader_refcnt);
+	else
+		return 1; /* No checks for non-atomic contexts for now */
+}
+
+static inline void warn_hotplug_unsafe(void)
+{
+	WARN_ONCE(1, "Must use get/put_online_cpus_atomic() to synchronize"
+		     " with CPU hotplug\n");
+}
+
+/*
+ * Check if the task (executing in atomic context) has the required protection
+ * against CPU hotplug, while accessing the specified cpumask.
+ */
+void check_hotplug_safe_cpumask(const struct cpumask *mask)
+{
+	if (!current_is_hotplug_safe(mask))
+		warn_hotplug_unsafe();
+}
+EXPORT_SYMBOL_GPL(check_hotplug_safe_cpumask);
+
+/*
+ * Similar to check_hotplug_safe_cpumask(), except that we don't complain
+ * if the task (executing in atomic context) is testing whether the CPU it
+ * is executing on is online or not.
+ *
+ * (A task executing with preemption disabled on a CPU, automatically prevents
+ *  offlining that CPU, irrespective of the actual implementation of CPU
+ *  offline. So we don't enforce holding of get_online_cpus_atomic() for that
+ *  case).
+ */
+void check_hotplug_safe_cpu(unsigned int cpu, const struct cpumask *mask)
+{
+	if(!current_is_hotplug_safe(mask) && cpu != smp_processor_id())
+		warn_hotplug_unsafe();
+}
+EXPORT_SYMBOL_GPL(check_hotplug_safe_cpu);
+
+static inline void atomic_reader_refcnt_inc(void)
+{
+	this_cpu_inc(atomic_reader_refcnt);
+}
+
+static inline void atomic_reader_refcnt_dec(void)
+{
+	this_cpu_dec(atomic_reader_refcnt);
+}
+
+#else
+
+static inline void atomic_reader_refcnt_inc(void)
+{
+}
+
+static inline void atomic_reader_refcnt_dec(void)
+{
+}
+
+#endif
+
 void get_online_cpus(void)
 {
 	might_sleep();
@@ -189,12 +275,15 @@ unsigned int get_online_cpus_atomic(void)
 	 * from going offline.
 	 */
 	preempt_disable();
+	atomic_reader_refcnt_inc();
+
 	return smp_processor_id();
 }
 EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
 
 void put_online_cpus_atomic(void)
 {
+	atomic_reader_refcnt_dec();
 	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(put_online_cpus_atomic);

[PATCH v2 05/45] CPU hotplug: Protect set_cpu_online() to avoid false-positives

[Srivatsa S. Bhat]

When bringing a secondary CPU online, the task running on the CPU coming up
sets itself in the cpu_online_mask. This is safe even though this task is not
the hotplug writer task.

But it is kinda hard to teach this to the CPU hotplug debug infrastructure,
and if we get it wrong, we risk making the debug code too lenient, risking
false-negatives.

Luckily, all architectures use set_cpu_online() to manipulate the
cpu_online_mask. So, to avoid false-positive warnings from the CPU hotplug
debug code, encapsulate the body of set_cpu_online() within
get/put_online_cpus_atomic().

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5297ec1..35e7115 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -832,10 +832,14 @@ void set_cpu_present(unsigned int cpu, bool present)
 
 void set_cpu_online(unsigned int cpu, bool online)
 {
+	get_online_cpus_atomic();
+
 	if (online)
 		cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
 	else
 		cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
+
+	put_online_cpus_atomic();
 }
 
 void set_cpu_active(unsigned int cpu, bool active)

[PATCH v2 06/45] CPU hotplug: Sprinkle debugging checks to catch locking bugs

[Srivatsa S. Bhat]

Now that we have a debug infrastructure in place to detect cases where
get/put_online_cpus_atomic() had to be used, add these checks at the
right spots to help catch places where we missed converting to the new
APIs.

Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Alex Shi <alex.shi@intel.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Joonsoo Kim <js1304@gmail.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpumask.h |   47 +++++++++++++++++++++++++++++++++++++++++++++--
 lib/cpumask.c           |    8 ++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 9197ca4..06d2c36 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -169,6 +169,7 @@ static inline unsigned int cpumask_any_but(const struct cpumask *mask,
  */
 static inline unsigned int cpumask_first(const struct cpumask *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
 	return find_first_bit(cpumask_bits(srcp), nr_cpumask_bits);
 }
 
@@ -184,6 +185,8 @@ static inline unsigned int cpumask_next(int n, const struct cpumask *srcp)
 	/* -1 is a legal arg here. */
 	if (n != -1)
 		cpumask_check(n);
+
+	check_hotplug_safe_cpumask(srcp);
 	return find_next_bit(cpumask_bits(srcp), nr_cpumask_bits, n+1);
 }
 
@@ -199,6 +202,8 @@ static inline unsigned int cpumask_next_zero(int n, const struct cpumask *srcp)
 	/* -1 is a legal arg here. */
 	if (n != -1)
 		cpumask_check(n);
+
+	check_hotplug_safe_cpumask(srcp);
 	return find_next_zero_bit(cpumask_bits(srcp), nr_cpumask_bits, n+1);
 }
 
@@ -288,8 +293,15 @@ static inline void cpumask_clear_cpu(int cpu, struct cpumask *dstp)
  *
  * No static inline type checking - see Subtlety (1) above.
  */
-#define cpumask_test_cpu(cpu, cpumask) \
-	test_bit(cpumask_check(cpu), cpumask_bits((cpumask)))
+#define cpumask_test_cpu(cpu, cpumask)				\
+({								\
+	int __ret;						\
+								\
+	check_hotplug_safe_cpu(cpu, cpumask);			\
+	__ret = test_bit(cpumask_check(cpu),			\
+				cpumask_bits((cpumask)));	\
+	__ret;							\
+})
 
 /**
  * cpumask_test_and_set_cpu - atomically test and set a cpu in a cpumask
@@ -349,6 +361,9 @@ static inline int cpumask_and(struct cpumask *dstp,
 			       const struct cpumask *src1p,
 			       const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	return bitmap_and(cpumask_bits(dstp), cpumask_bits(src1p),
 				       cpumask_bits(src2p), nr_cpumask_bits);
 }
@@ -362,6 +377,9 @@ static inline int cpumask_and(struct cpumask *dstp,
 static inline void cpumask_or(struct cpumask *dstp, const struct cpumask *src1p,
 			      const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	bitmap_or(cpumask_bits(dstp), cpumask_bits(src1p),
 				      cpumask_bits(src2p), nr_cpumask_bits);
 }
@@ -376,6 +394,9 @@ static inline void cpumask_xor(struct cpumask *dstp,
 			       const struct cpumask *src1p,
 			       const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	bitmap_xor(cpumask_bits(dstp), cpumask_bits(src1p),
 				       cpumask_bits(src2p), nr_cpumask_bits);
 }
@@ -392,6 +413,9 @@ static inline int cpumask_andnot(struct cpumask *dstp,
 				  const struct cpumask *src1p,
 				  const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	return bitmap_andnot(cpumask_bits(dstp), cpumask_bits(src1p),
 					  cpumask_bits(src2p), nr_cpumask_bits);
 }
@@ -404,6 +428,8 @@ static inline int cpumask_andnot(struct cpumask *dstp,
 static inline void cpumask_complement(struct cpumask *dstp,
 				      const struct cpumask *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
+
 	bitmap_complement(cpumask_bits(dstp), cpumask_bits(srcp),
 					      nr_cpumask_bits);
 }
@@ -416,6 +442,9 @@ static inline void cpumask_complement(struct cpumask *dstp,
 static inline bool cpumask_equal(const struct cpumask *src1p,
 				const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	return bitmap_equal(cpumask_bits(src1p), cpumask_bits(src2p),
 						 nr_cpumask_bits);
 }
@@ -428,6 +457,10 @@ static inline bool cpumask_equal(const struct cpumask *src1p,
 static inline bool cpumask_intersects(const struct cpumask *src1p,
 				     const struct cpumask *src2p)
 {
+
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	return bitmap_intersects(cpumask_bits(src1p), cpumask_bits(src2p),
 						      nr_cpumask_bits);
 }
@@ -442,6 +475,9 @@ static inline bool cpumask_intersects(const struct cpumask *src1p,
 static inline int cpumask_subset(const struct cpumask *src1p,
 				 const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	return bitmap_subset(cpumask_bits(src1p), cpumask_bits(src2p),
 						  nr_cpumask_bits);
 }
@@ -470,6 +506,12 @@ static inline bool cpumask_full(const struct cpumask *srcp)
  */
 static inline unsigned int cpumask_weight(const struct cpumask *srcp)
 {
+	/*
+	 * Often, we just want to have a rough estimate of the number of
+	 * online CPUs, without going to the trouble of synchronizing with
+	 * CPU hotplug. So don't invoke check_hotplug_safe_cpumask() here.
+	 */
+
 	return bitmap_weight(cpumask_bits(srcp), nr_cpumask_bits);
 }
 
@@ -507,6 +549,7 @@ static inline void cpumask_shift_left(struct cpumask *dstp,
 static inline void cpumask_copy(struct cpumask *dstp,
 				const struct cpumask *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
 	bitmap_copy(cpumask_bits(dstp), cpumask_bits(srcp), nr_cpumask_bits);
 }
 
diff --git a/lib/cpumask.c b/lib/cpumask.c
index d327b87..481df57 100644
--- a/lib/cpumask.c
+++ b/lib/cpumask.c
@@ -7,12 +7,14 @@
 
 int __first_cpu(const cpumask_t *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
 	return min_t(int, NR_CPUS, find_first_bit(srcp->bits, NR_CPUS));
 }
 EXPORT_SYMBOL(__first_cpu);
 
 int __next_cpu(int n, const cpumask_t *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
 	return min_t(int, NR_CPUS, find_next_bit(srcp->bits, NR_CPUS, n+1));
 }
 EXPORT_SYMBOL(__next_cpu);
@@ -20,6 +22,7 @@ EXPORT_SYMBOL(__next_cpu);
 #if NR_CPUS > 64
 int __next_cpu_nr(int n, const cpumask_t *srcp)
 {
+	check_hotplug_safe_cpumask(srcp);
 	return min_t(int, nr_cpu_ids,
 				find_next_bit(srcp->bits, nr_cpu_ids, n+1));
 }
@@ -37,6 +40,9 @@ EXPORT_SYMBOL(__next_cpu_nr);
 int cpumask_next_and(int n, const struct cpumask *src1p,
 		     const struct cpumask *src2p)
 {
+	check_hotplug_safe_cpumask(src1p);
+	check_hotplug_safe_cpumask(src2p);
+
 	while ((n = cpumask_next(n, src1p)) < nr_cpu_ids)
 		if (cpumask_test_cpu(n, src2p))
 			break;
@@ -57,6 +63,8 @@ int cpumask_any_but(const struct cpumask *mask, unsigned int cpu)
 	unsigned int i;
 
 	cpumask_check(cpu);
+	check_hotplug_safe_cpumask(mask);
+
 	for_each_cpu(i, mask)
 		if (i != cpu)
 			break;

[PATCH v2 07/45] CPU hotplug: Expose the new debug config option

[Srivatsa S. Bhat]

Now that we have all the pieces of the CPU hotplug debug infrastructure
in place, expose the feature by growing a new Kconfig option,
CONFIG_DEBUG_HOTPLUG_CPU.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Paul E. McKenney" <paul.mckenney@linaro.org>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 lib/Kconfig.debug |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 566cf2b..ec6be74 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -512,6 +512,14 @@ config DEBUG_PREEMPT
 	  if kernel code uses it in a preemption-unsafe way. Also, the kernel
 	  will detect preemption count underflows.
 
+config DEBUG_HOTPLUG_CPU
+	bool "Debug CPU hotplug"
+	depends on HOTPLUG_CPU
+	help
+	  If you say Y here, the kernel will check all the accesses of
+	  cpu_online_mask from atomic contexts, and will print warnings if
+	  the task lacks appropriate synchronization with CPU hotplug.
+
 config DEBUG_RT_MUTEXES
 	bool "RT Mutex debugging, deadlock detection"
 	depends on DEBUG_KERNEL && RT_MUTEXES

[PATCH v2 08/45] CPU hotplug: Convert preprocessor macros to static inline functions

[Srivatsa S. Bhat]

Convert the macros in the CPU hotplug code to static inline C functions.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/cpu.h |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index a57b25a..85431a0 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -202,10 +202,8 @@ static inline void cpu_hotplug_driver_unlock(void)
 
 #else		/* CONFIG_HOTPLUG_CPU */
 
-#define get_online_cpus()	do { } while (0)
-#define put_online_cpus()	do { } while (0)
-#define cpu_hotplug_disable()	do { } while (0)
-#define cpu_hotplug_enable()	do { } while (0)
+static inline void get_online_cpus(void) {}
+static inline void put_online_cpus(void) {}
 
 static inline unsigned int get_online_cpus_atomic(void)
 {
@@ -222,6 +220,9 @@ static inline void put_online_cpus_atomic(void)
 	preempt_enable();
 }
 
+static inline void cpu_hotplug_disable(void) {}
+static inline void cpu_hotplug_enable(void) {}
+
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })

[PATCH v2 09/45] smp: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Wang YanQing <udknight@gmail.com>
Cc: Shaohua Li <shli@fusionio.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/smp.c |   52 ++++++++++++++++++++++++++++++----------------------
 1 file changed, 30 insertions(+), 22 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 4dba0f7..1f36d6d 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -232,7 +232,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 	 * prevent preemption and reschedule on another processor,
 	 * as well as CPU removal
 	 */
-	this_cpu = get_cpu();
+	this_cpu = get_online_cpus_atomic();
 
 	/*
 	 * Can deadlock when called with interrupts disabled.
@@ -264,7 +264,7 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info,
 		}
 	}
 
-	put_cpu();
+	put_online_cpus_atomic();
 
 	return err;
 }
@@ -294,7 +294,7 @@ int smp_call_function_any(const struct cpumask *mask,
 	int ret;
 
 	/* Try for same CPU (cheapest) */
-	cpu = get_cpu();
+	cpu = get_online_cpus_atomic();
 	if (cpumask_test_cpu(cpu, mask))
 		goto call;
 
@@ -310,7 +310,7 @@ int smp_call_function_any(const struct cpumask *mask,
 	cpu = cpumask_any_and(mask, cpu_online_mask);
 call:
 	ret = smp_call_function_single(cpu, func, info, wait);
-	put_cpu();
+	put_online_cpus_atomic();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(smp_call_function_any);
@@ -331,7 +331,8 @@ void __smp_call_function_single(int cpu, struct call_single_data *csd,
 	unsigned int this_cpu;
 	unsigned long flags;
 
-	this_cpu = get_cpu();
+	this_cpu = get_online_cpus_atomic();
+
 	/*
 	 * Can deadlock when called with interrupts disabled.
 	 * We allow cpu's that are not yet online though, as no one else can
@@ -349,7 +350,8 @@ void __smp_call_function_single(int cpu, struct call_single_data *csd,
 		csd_lock(csd);
 		generic_exec_single(cpu, csd, wait);
 	}
-	put_cpu();
+
+	put_online_cpus_atomic();
 }
 
 /**
@@ -370,7 +372,9 @@ void smp_call_function_many(const struct cpumask *mask,
 			    smp_call_func_t func, void *info, bool wait)
 {
 	struct call_function_data *cfd;
-	int cpu, next_cpu, this_cpu = smp_processor_id();
+	int cpu, next_cpu, this_cpu;
+
+	this_cpu = get_online_cpus_atomic();
 
 	/*
 	 * Can deadlock when called with interrupts disabled.
@@ -388,7 +392,7 @@ void smp_call_function_many(const struct cpumask *mask,
 
 	/* No online cpus?  We're done. */
 	if (cpu >= nr_cpu_ids)
-		return;
+		goto out;
 
 	/* Do we have another CPU which isn't us? */
 	next_cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
@@ -398,7 +402,7 @@ void smp_call_function_many(const struct cpumask *mask,
 	/* Fastpath: do that cpu by itself. */
 	if (next_cpu >= nr_cpu_ids) {
 		smp_call_function_single(cpu, func, info, wait);
-		return;
+		goto out;
 	}
 
 	cfd = &__get_cpu_var(cfd_data);
@@ -408,7 +412,7 @@ void smp_call_function_many(const struct cpumask *mask,
 
 	/* Some callers race with other cpus changing the passed mask */
 	if (unlikely(!cpumask_weight(cfd->cpumask)))
-		return;
+		goto out;
 
 	/*
 	 * After we put an entry into the list, cfd->cpumask may be cleared
@@ -443,6 +447,9 @@ void smp_call_function_many(const struct cpumask *mask,
 			csd_lock_wait(csd);
 		}
 	}
+
+out:
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL(smp_call_function_many);
 
@@ -463,9 +470,9 @@ EXPORT_SYMBOL(smp_call_function_many);
  */
 int smp_call_function(smp_call_func_t func, void *info, int wait)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 	smp_call_function_many(cpu_online_mask, func, info, wait);
-	preempt_enable();
+	put_online_cpus_atomic();
 
 	return 0;
 }
@@ -565,12 +572,12 @@ int on_each_cpu(void (*func) (void *info), void *info, int wait)
 	unsigned long flags;
 	int ret = 0;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	ret = smp_call_function(func, info, wait);
 	local_irq_save(flags);
 	func(info);
 	local_irq_restore(flags);
-	preempt_enable();
+	put_online_cpus_atomic();
 	return ret;
 }
 EXPORT_SYMBOL(on_each_cpu);
@@ -592,7 +599,7 @@ EXPORT_SYMBOL(on_each_cpu);
 void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
 			void *info, bool wait)
 {
-	int cpu = get_cpu();
+	unsigned int cpu = get_online_cpus_atomic();
 
 	smp_call_function_many(mask, func, info, wait);
 	if (cpumask_test_cpu(cpu, mask)) {
@@ -600,7 +607,7 @@ void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
 		func(info);
 		local_irq_enable();
 	}
-	put_cpu();
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL(on_each_cpu_mask);
 
@@ -625,8 +632,9 @@ EXPORT_SYMBOL(on_each_cpu_mask);
  * The function might sleep if the GFP flags indicates a non
  * atomic allocation is allowed.
  *
- * Preemption is disabled to protect against CPUs going offline but not online.
- * CPUs going online during the call will not be seen or sent an IPI.
+ * We use get/put_online_cpus_atomic() to protect against CPUs going
+ * offline but not online. CPUs going online during the call will
+ * not be seen or sent an IPI.
  *
  * You must not call this function with disabled interrupts or
  * from a hardware interrupt handler or from a bottom half handler.
@@ -641,26 +649,26 @@ void on_each_cpu_cond(bool (*cond_func)(int cpu, void *info),
 	might_sleep_if(gfp_flags & __GFP_WAIT);
 
 	if (likely(zalloc_cpumask_var(&cpus, (gfp_flags|__GFP_NOWARN)))) {
-		preempt_disable();
+		get_online_cpus_atomic();
 		for_each_online_cpu(cpu)
 			if (cond_func(cpu, info))
 				cpumask_set_cpu(cpu, cpus);
 		on_each_cpu_mask(cpus, func, info, wait);
-		preempt_enable();
+		put_online_cpus_atomic();
 		free_cpumask_var(cpus);
 	} else {
 		/*
 		 * No free cpumask, bother. No matter, we'll
 		 * just have to IPI them one by one.
 		 */
-		preempt_disable();
+		get_online_cpus_atomic();
 		for_each_online_cpu(cpu)
 			if (cond_func(cpu, info)) {
 				ret = smp_call_function_single(cpu, func,
 								info, wait);
 				WARN_ON_ONCE(!ret);
 			}
-		preempt_enable();
+		put_online_cpus_atomic();
 	}
 }
 EXPORT_SYMBOL(on_each_cpu_cond);

[PATCH v2 10/45] sched/core: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/core.c |   23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 195658b..accd550 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1160,11 +1160,11 @@ void kick_process(struct task_struct *p)
 {
 	int cpu;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpu = task_cpu(p);
 	if ((cpu != smp_processor_id()) && task_curr(p))
 		smp_send_reschedule(cpu);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL_GPL(kick_process);
 #endif /* CONFIG_SMP */
@@ -1172,6 +1172,9 @@ EXPORT_SYMBOL_GPL(kick_process);
 #ifdef CONFIG_SMP
 /*
  * ->cpus_allowed is protected by both rq->lock and p->pi_lock
+ *
+ *  Must be called within get/put_online_cpus_atomic(), to prevent
+ *  CPUs from going offline from under us.
  */
 static int select_fallback_rq(int cpu, struct task_struct *p)
 {
@@ -1245,6 +1248,9 @@ out:
 
 /*
  * The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
+ *
+ * Must be called within get/put_online_cpus_atomic(), to prevent
+ * CPUs from going offline from under us.
  */
 static inline
 int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
@@ -1489,6 +1495,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	unsigned long flags;
 	int cpu, success = 0;
 
+	get_online_cpus_atomic();
+
 	smp_wmb();
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
 	if (!(p->state & state))
@@ -1531,6 +1539,7 @@ stat:
 out:
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
 
+	put_online_cpus_atomic();
 	return success;
 }
 
@@ -1753,6 +1762,8 @@ void wake_up_new_task(struct task_struct *p)
 	unsigned long flags;
 	struct rq *rq;
 
+	get_online_cpus_atomic();
+
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
 #ifdef CONFIG_SMP
 	/*
@@ -1773,6 +1784,8 @@ void wake_up_new_task(struct task_struct *p)
 		p->sched_class->task_woken(rq, p);
 #endif
 	task_rq_unlock(rq, p, &flags);
+
+	put_online_cpus_atomic();
 }
 
 #ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -3886,6 +3899,8 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
 	unsigned long flags;
 	int yielded = 0;
 
+	get_online_cpus_atomic();
+
 	local_irq_save(flags);
 	rq = this_rq();
 
@@ -3931,6 +3946,8 @@ out_unlock:
 out_irq:
 	local_irq_restore(flags);
 
+	put_online_cpus_atomic();
+
 	if (yielded > 0)
 		schedule();
 
@@ -4331,9 +4348,11 @@ static int migration_cpu_stop(void *data)
 	 * The original target cpu might have gone down and we might
 	 * be on another cpu but it doesn't matter.
 	 */
+	get_online_cpus_atomic();
 	local_irq_disable();
 	__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
 	local_irq_enable();
+	put_online_cpus_atomic();
 	return 0;
 }
 

[PATCH v2 11/45] migration: Use raw_spin_lock/unlock since interrupts are already disabled

[Srivatsa S. Bhat]

We need not use the raw_spin_lock_irqsave/restore primitives because
all CPU_DYING notifiers run with interrupts disabled. So just use
raw_spin_lock/unlock.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/core.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index accd550..ff26f54 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4682,14 +4682,14 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
 	case CPU_DYING:
 		sched_ttwu_pending();
 		/* Update our root-domain */
-		raw_spin_lock_irqsave(&rq->lock, flags);
+		raw_spin_lock(&rq->lock); /* IRQs already disabled */
 		if (rq->rd) {
 			BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
 			set_rq_offline(rq);
 		}
 		migrate_tasks(cpu);
 		BUG_ON(rq->nr_running != 1); /* the migration thread */
-		raw_spin_unlock_irqrestore(&rq->lock, flags);
+		raw_spin_unlock(&rq->lock);
 		break;
 
 	case CPU_DEAD:

[PATCH v2 12/45] sched/fair: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/fair.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c0ac2c3..88f056e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3338,7 +3338,8 @@ done:
  *
  * Returns the target CPU number, or the same CPU if no balancing is needed.
  *
- * preempt must be disabled.
+ * Must be called within get/put_online_cpus_atomic(), to prevent CPUs
+ * from going offline from under us.
  */
 static int
 select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
@@ -5267,6 +5268,8 @@ void idle_balance(int this_cpu, struct rq *this_rq)
 	raw_spin_unlock(&this_rq->lock);
 
 	update_blocked_averages(this_cpu);
+
+	get_online_cpus_atomic();
 	rcu_read_lock();
 	for_each_domain(this_cpu, sd) {
 		unsigned long interval;
@@ -5290,6 +5293,7 @@ void idle_balance(int this_cpu, struct rq *this_rq)
 		}
 	}
 	rcu_read_unlock();
+	put_online_cpus_atomic();
 
 	raw_spin_lock(&this_rq->lock);
 
@@ -5316,6 +5320,7 @@ static int active_load_balance_cpu_stop(void *data)
 	struct rq *target_rq = cpu_rq(target_cpu);
 	struct sched_domain *sd;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irq(&busiest_rq->lock);
 
 	/* make sure the requested cpu hasn't gone down in the meantime */
@@ -5367,6 +5372,7 @@ static int active_load_balance_cpu_stop(void *data)
 out_unlock:
 	busiest_rq->active_balance = 0;
 	raw_spin_unlock_irq(&busiest_rq->lock);
+	put_online_cpus_atomic();
 	return 0;
 }
 
@@ -5527,6 +5533,7 @@ static void rebalance_domains(int cpu, enum cpu_idle_type idle)
 
 	update_blocked_averages(cpu);
 
+	get_online_cpus_atomic();
 	rcu_read_lock();
 	for_each_domain(cpu, sd) {
 		if (!(sd->flags & SD_LOAD_BALANCE))
@@ -5575,6 +5582,7 @@ out:
 			break;
 	}
 	rcu_read_unlock();
+	put_online_cpus_atomic();
 
 	/*
 	 * next_balance will be updated only when there is a need.
@@ -5706,6 +5714,7 @@ static void run_rebalance_domains(struct softirq_action *h)
 	enum cpu_idle_type idle = this_rq->idle_balance ?
 						CPU_IDLE : CPU_NOT_IDLE;
 
+	get_online_cpus_atomic();
 	rebalance_domains(this_cpu, idle);
 
 	/*
@@ -5714,6 +5723,7 @@ static void run_rebalance_domains(struct softirq_action *h)
 	 * stopped.
 	 */
 	nohz_idle_balance(this_cpu, idle);
+	put_online_cpus_atomic();
 }
 
 static inline int on_null_domain(int cpu)
@@ -5731,8 +5741,10 @@ void trigger_load_balance(struct rq *rq, int cpu)
 	    likely(!on_null_domain(cpu)))
 		raise_softirq(SCHED_SOFTIRQ);
 #ifdef CONFIG_NO_HZ_COMMON
+	get_online_cpus_atomic();
 	if (nohz_kick_needed(rq, cpu) && likely(!on_null_domain(cpu)))
 		nohz_balancer_kick(cpu);
+	put_online_cpus_atomic();
 #endif
 }
 

[PATCH v2 13/45] timer: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/timer.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/timer.c b/kernel/timer.c
index 15ffdb3..5db594c 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -729,6 +729,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 	timer_stats_timer_set_start_info(timer);
 	BUG_ON(!timer->function);
 
+	get_online_cpus_atomic();
 	base = lock_timer_base(timer, &flags);
 
 	ret = detach_if_pending(timer, base, false);
@@ -768,6 +769,7 @@ __mod_timer(struct timer_list *timer, unsigned long expires,
 
 out_unlock:
 	spin_unlock_irqrestore(&base->lock, flags);
+	put_online_cpus_atomic();
 
 	return ret;
 }
@@ -926,6 +928,7 @@ void add_timer_on(struct timer_list *timer, int cpu)
 
 	timer_stats_timer_set_start_info(timer);
 	BUG_ON(timer_pending(timer) || !timer->function);
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&base->lock, flags);
 	timer_set_base(timer, base);
 	debug_activate(timer, timer->expires);
@@ -940,6 +943,7 @@ void add_timer_on(struct timer_list *timer, int cpu)
 	 */
 	wake_up_nohz_cpu(cpu);
 	spin_unlock_irqrestore(&base->lock, flags);
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL_GPL(add_timer_on);
 

[PATCH v2 14/45] sched/rt: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/sched/rt.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 01970c8..03d9f38 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -6,6 +6,7 @@
 #include "sched.h"
 
 #include <linux/slab.h>
+#include <linux/cpu.h>
 
 int sched_rr_timeslice = RR_TIMESLICE;
 
@@ -28,7 +29,9 @@ static enum hrtimer_restart sched_rt_period_timer(struct hrtimer *timer)
 		if (!overrun)
 			break;
 
+		get_online_cpus_atomic();
 		idle = do_sched_rt_period_timer(rt_b, overrun);
+		put_online_cpus_atomic();
 	}
 
 	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
@@ -547,6 +550,7 @@ static int do_balance_runtime(struct rt_rq *rt_rq)
 	int i, weight, more = 0;
 	u64 rt_period;
 
+	get_online_cpus_atomic();
 	weight = cpumask_weight(rd->span);
 
 	raw_spin_lock(&rt_b->rt_runtime_lock);
@@ -588,6 +592,7 @@ next:
 		raw_spin_unlock(&iter->rt_runtime_lock);
 	}
 	raw_spin_unlock(&rt_b->rt_runtime_lock);
+	put_online_cpus_atomic();
 
 	return more;
 }
@@ -1168,6 +1173,10 @@ static void yield_task_rt(struct rq *rq)
 #ifdef CONFIG_SMP
 static int find_lowest_rq(struct task_struct *task);
 
+/*
+ * Must be called within get/put_online_cpus_atomic(), to prevent CPUs
+ * from going offline from under us.
+ */
 static int
 select_task_rq_rt(struct task_struct *p, int sd_flag, int flags)
 {
@@ -1561,6 +1570,8 @@ retry:
 		return 0;
 	}
 
+	get_online_cpus_atomic();
+
 	/* We might release rq lock */
 	get_task_struct(next_task);
 
@@ -1611,6 +1622,7 @@ retry:
 out:
 	put_task_struct(next_task);
 
+	put_online_cpus_atomic();
 	return ret;
 }
 
@@ -1630,6 +1642,7 @@ static int pull_rt_task(struct rq *this_rq)
 	if (likely(!rt_overloaded(this_rq)))
 		return 0;
 
+	get_online_cpus_atomic();
 	for_each_cpu(cpu, this_rq->rd->rto_mask) {
 		if (this_cpu == cpu)
 			continue;
@@ -1695,6 +1708,7 @@ skip:
 		double_unlock_balance(this_rq, src_rq);
 	}
 
+	put_online_cpus_atomic();
 	return ret;
 }
 

[PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
while being protected by a spinlock. Use the get/put_online_cpus_atomic()
APIs to prevent CPUs from going offline, while invoking from atomic context.

Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/rcutree.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index cf3adc6..caeed1a 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2107,6 +2107,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 			rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
 			continue;
 		}
+
+		get_online_cpus_atomic();
 		cpu = rnp->grplo;
 		bit = 1;
 		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
@@ -2114,6 +2116,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 			    f(per_cpu_ptr(rsp->rda, cpu)))
 				mask |= bit;
 		}
+		put_online_cpus_atomic();
+
 		if (mask != 0) {
 
 			/* rcu_report_qs_rnp() releases rnp->lock. */

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 01:57:55AM +0530, Srivatsa S. Bhat wrote:
> Once stop_machine() is gone from the CPU offline path, we won't be able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.
> 
> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
> while being protected by a spinlock. Use the get/put_online_cpus_atomic()
> APIs to prevent CPUs from going offline, while invoking from atomic context.

I am not completely sure that this is needed.  Here is a (quite possibly
flawed) argument for its not being needed:

o	rcu_gp_init() holds off CPU-hotplug operations during
	grace-period initialization.  Therefore, RCU will avoid
	looking for quiescent states from CPUs that were offline
	(and thus in an extended quiescent state) at the beginning
	of the grace period.

o	If force_qs_rnp() is looking for a quiescent state from
	a given CPU, and if it senses that CPU as being offline,
	then even without synchronization we know that the CPU
	was offline some time during the current grace period.

	After all, it was online at the beginning of the grace
	period (otherwise, we would not be looking at it at all),
	and our later sampling of its state must have therefore
	happened after the start of the grace period.  Given that
	the grace period has not yet ended, it also has to happened
	before the end of the grace period.

o	Therefore, we should be able to sample the offline state
	without synchronization.

Possible flaws in this argument:  memory ordering, oddnesses in
the sampling and updates of the cpumask recording which CPUs are
online, and so on.

Thoughts?

							Thanx, Paul

> Cc: Dipankar Sarma <dipankar@in.ibm.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> ---
> 
>  kernel/rcutree.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index cf3adc6..caeed1a 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2107,6 +2107,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
>  			rcu_initiate_boost(rnp, flags); /* releases rnp->lock */
>  			continue;
>  		}
> +
> +		get_online_cpus_atomic();
>  		cpu = rnp->grplo;
>  		bit = 1;
>  		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
> @@ -2114,6 +2116,8 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
>  			    f(per_cpu_ptr(rsp->rda, cpu)))
>  				mask |= bit;
>  		}
> +		put_online_cpus_atomic();
> +
>  		if (mask != 0) {
> 
>  			/* rcu_report_qs_rnp() releases rnp->lock. */
> 

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 03:30 AM, Paul E. McKenney wrote:
> On Wed, Jun 26, 2013 at 01:57:55AM +0530, Srivatsa S. Bhat wrote:
>> Once stop_machine() is gone from the CPU offline path, we won't be able
>> to depend on disabling preemption to prevent CPUs from going offline
>> from under us.
>>
>> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
>> while being protected by a spinlock. Use the get/put_online_cpus_atomic()
>> APIs to prevent CPUs from going offline, while invoking from atomic context.
> 
> I am not completely sure that this is needed.  Here is a (quite possibly
> flawed) argument for its not being needed:
> 
> o	rcu_gp_init() holds off CPU-hotplug operations during
> 	grace-period initialization.  Therefore, RCU will avoid
> 	looking for quiescent states from CPUs that were offline
> 	(and thus in an extended quiescent state) at the beginning
> 	of the grace period.
> 
> o	If force_qs_rnp() is looking for a quiescent state from
> 	a given CPU, and if it senses that CPU as being offline,
> 	then even without synchronization we know that the CPU
> 	was offline some time during the current grace period.
> 
> 	After all, it was online at the beginning of the grace
> 	period (otherwise, we would not be looking at it at all),
> 	and our later sampling of its state must have therefore
> 	happened after the start of the grace period.  Given that
> 	the grace period has not yet ended, it also has to happened
> 	before the end of the grace period.
> 
> o	Therefore, we should be able to sample the offline state
> 	without synchronization.
>

Thanks a lot for explaining the synchronization design in detail, Paul!
I agree that get/put_online_cpus_atomic() is not necessary here.

Regarding the debug checks under CONFIG_DEBUG_HOTPLUG_CPU, to avoid
false-positives, I'm thinking of introducing a few _nocheck() variants,
on a case-by-case basis, like cpu_is_offline_nocheck() (useful here in RCU)
and for_each_online_cpu_nocheck() (useful in percpu-counter code, as
pointed out by Tejun Heo). These fine synchronization details are kinda
hard to encapsulate in that debug logic, so we can use the _nocheck()
variants here to avoid getting splats when running with DEBUG_HOTPLUG_CPU
enabled.

Regards,
Srivatsa S. Bhat

RE: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[David Laight]

> Once stop_machine() is gone from the CPU offline path, we won't be =
able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.

Could you use an rcu-like sequence so that disabling pre-emption
would be enough?

Something like rebuilding the cpu list, then forcing yourself
to run on all the cpu.

That would be far less intrusive.

	David

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 03:29:40PM +0100, David Laight wrote:
> > Once stop_machine() is gone from the CPU offline path, we won't be able
> > to depend on disabling preemption to prevent CPUs from going offline
> > from under us.
> 
> Could you use an rcu-like sequence so that disabling pre-emption
> would be enough?
> 
> Something like rebuilding the cpu list, then forcing yourself
> to run on all the cpu.
> 
> That would be far less intrusive.

It would also increase the latency of CPU-hotunplug operations.

							Thanx, Paul

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Steven Rostedt]

On Wed, 2013-06-26 at 07:34 -0700, Paul E. McKenney wrote:
> On Wed, Jun 26, 2013 at 03:29:40PM +0100, David Laight wrote:
> > > Once stop_machine() is gone from the CPU offline path, we won't be able
> > > to depend on disabling preemption to prevent CPUs from going offline
> > > from under us.
> > 
> > Could you use an rcu-like sequence so that disabling pre-emption
> > would be enough?
> > 
> > Something like rebuilding the cpu list, then forcing yourself
> > to run on all the cpu.
> > 
> > That would be far less intrusive.
> 
> It would also increase the latency of CPU-hotunplug operations.
> 

Is that a big deal?

-- Steve

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Tejun Heo]

On Wed, Jun 26, 2013 at 10:51:11AM -0400, Steven Rostedt wrote:
> > It would also increase the latency of CPU-hotunplug operations.
> 
> Is that a big deal?

I thought that was the whole deal with this patchset - making cpu
hotunplugs lighter and faster mostly for powersaving.  That said, just
removing stop_machine call would be a pretty good deal and I don't
know how meaningful reducing CPU hotunplug latency is.  Srivatsa?

Thanks.

-- 
tejun

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Steven Rostedt]

On Wed, 2013-06-26 at 08:21 -0700, Tejun Heo wrote:
> On Wed, Jun 26, 2013 at 10:51:11AM -0400, Steven Rostedt wrote:
> > > It would also increase the latency of CPU-hotunplug operations.
> > 
> > Is that a big deal?
> 
> I thought that was the whole deal with this patchset - making cpu
> hotunplugs lighter and faster mostly for powersaving.  That said, just
> removing stop_machine call would be a pretty good deal and I don't
> know how meaningful reducing CPU hotunplug latency is.  Srivatsa?

I thought the whole deal with this patchset was to remove stop_machine
from CPU hotplug. Why halt all CPUs just to remove one? stomp_machine()
is extremely intrusive for the entire system, where as one CPU making
sure all CPUs schedule isn't very intrusive at all.

I didn't think the idea of this patch set was to make CPU hotplug
faster, just less intrusive to the system.

-- Steve

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Tejun Heo]

Hello,

On Wed, Jun 26, 2013 at 11:33:43AM -0400, Steven Rostedt wrote:
> I thought the whole deal with this patchset was to remove stop_machine
> from CPU hotplug. Why halt all CPUs just to remove one? stomp_machine()
> is extremely intrusive for the entire system, where as one CPU making
> sure all CPUs schedule isn't very intrusive at all.
> 
> I didn't think the idea of this patch set was to make CPU hotplug
> faster, just less intrusive to the system.

Yeap, removal of stop_machine is a great improvement in itself.  ISTR
mentions of hot-unplug latency but I could be mistaken.  Srivatsa, can
you please chime in on that?

Thanks.

-- 
tejun

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 10:59 PM, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jun 26, 2013 at 11:33:43AM -0400, Steven Rostedt wrote:
>> I thought the whole deal with this patchset was to remove stop_machine
>> from CPU hotplug. Why halt all CPUs just to remove one? stomp_machine()
>> is extremely intrusive for the entire system, where as one CPU making
>> sure all CPUs schedule isn't very intrusive at all.
>>
>> I didn't think the idea of this patch set was to make CPU hotplug
>> faster, just less intrusive to the system.
> 
> Yeap, removal of stop_machine is a great improvement in itself.

Absolutely. To make hotplug less intrusive on the system.

>  ISTR
> mentions of hot-unplug latency but I could be mistaken.  Srivatsa, can
> you please chime in on that?
> 

Yes, we were discussing hot-unplug latency for use-cases such as
suspend/resume. We didn't want to make those operations slower in the
process of removing stop_machine() from hotplug.

Regards,
Srivatsa S. Bhat

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Tejun Heo]

Hey,

On Wed, Jun 26, 2013 at 11:58:48PM +0530, Srivatsa S. Bhat wrote:
> Yes, we were discussing hot-unplug latency for use-cases such as
> suspend/resume. We didn't want to make those operations slower in the
> process of removing stop_machine() from hotplug.

Can you please explain why tho?  How much would it help in terms of
power-saving?  Or are there other issues in taking longer to shut down
cpus?

Thanks.

-- 
tejun

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/27/2013 03:04 AM, Tejun Heo wrote:
> Hey,
> 
> On Wed, Jun 26, 2013 at 11:58:48PM +0530, Srivatsa S. Bhat wrote:
>> Yes, we were discussing hot-unplug latency for use-cases such as
>> suspend/resume. We didn't want to make those operations slower in the
>> process of removing stop_machine() from hotplug.
> 
> Can you please explain why tho?

Sure.

>  How much would it help in terms of
> power-saving?  Or are there other issues in taking longer to shut down
> cpus?
> 

Basically, power-management techniques (like suspend/resume as used on
smartphones etc) are implemented using a controlling algorithm which
controls the aggressiveness of power-management depending on the load
on the system.

Today, the cpuidle subsystem in Linux handles cpuidle transitions using
that technique, so I'll explain using that as an example. Similar
strategies are used for suspend/resume also.

For every cpuidle state, we have atleast 2 parameters that determine
how usable it is - 1. exit latency 2. target residency.

The exit latency captures the amount of time it takes to come out of
the power-saving state, from the time you asked it to come out. This
is an estimate of how "deep" the power-saving state is. The deeper it
is, the longer it takes to come out. (And of course, deeper states give
higher power-savings).

The target residency is a value which represents the break-even for
that state. It tells us how long we should stay in that idle state
(at a minimum) before we ask it to come out, to actually get some good
power-savings out of that state. (Note that entry and exit to an idle
state itself consumes some power, so there won't be any power-savings
if we keep entering and exiting a deep state without staying in that
state long enough).

Once the idle states are characterized like this, the cpuidle subsystem
uses a cpuidle "governor" to actually decide which of the states to
enter, given an idle period of time. The governor keeps track of the
load on the system and predicts the length of the next idle period, and
based on that prediction, it selects the idle state whose characteristics
(exit latency/target residency) match the predicted idle time closely.

So as we can see, the "deeper" the idle state, the lower its chance of
getting used during regular workload runs, because it is deemed too costly.
So entry/exit latency of an idle state is a very important aspect which
determines how often you can use that state. Ideally we want states which
are "deep" in the sense that they give huge power-savings, but "light"
in the sense that their entry/exit latencies are low. Such states give
us the best benefits, since we can use them aggressively.

Now a similar argument applies for suspend/resume (on smartphones etc).
Suspend/resume already gives good power-savings. But if we make it slower,
the chances of using it profitably begins to reduce. That's why, IMHO,
its important to keep a check on the latency of CPU hotplug and reduce
it if possible. And that becomes even more important as systems keep
sporting more and more CPUs.

Regards,
Srivatsa S. Bhat

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 08:51 PM, Tejun Heo wrote:
> On Wed, Jun 26, 2013 at 10:51:11AM -0400, Steven Rostedt wrote:
>>> It would also increase the latency of CPU-hotunplug operations.
>>
>> Is that a big deal?
> 
> I thought that was the whole deal with this patchset - making cpu
> hotunplugs lighter and faster mostly for powersaving.  That said, just
> removing stop_machine call would be a pretty good deal and I don't
> know how meaningful reducing CPU hotunplug latency is.  Srivatsa?
> 

Keeping the hotunplug latency is important for suspend/resume, where
we take all non-boot CPUs in a loop. That's an interesting use-case
where intrusiveness doesn't matter much, but latency does. So yes,
making CPU hotplug faster is also one of the goals of this patchset.

Regards,
Srivatsa S. Bhat

RE: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[David Laight]

> >>> It would also increase the latency of CPU-hotunplug operations.
> >>
> >> Is that a big deal?
> >
> > I thought that was the whole deal with this patchset - making cpu
> > hotunplugs lighter and faster mostly for powersaving.  That said, =
just
> > removing stop_machine call would be a pretty good deal and I don't
> > know how meaningful reducing CPU hotunplug latency is.  Srivatsa?
> >
>=20
> Keeping the hotunplug latency is important for suspend/resume, where
> we take all non-boot CPUs in a loop. That's an interesting use-case
> where intrusiveness doesn't matter much, but latency does. So yes,
> making CPU hotplug faster is also one of the goals of this patchset.

If you are removing all but one of the cpu, the you only need
one rcu cycle (remove everything from the list first).

I'd also guess that you can't suspend a cpu until you can sleep
the process that is running on it - so if a process has pre-emption
disabled you aren't going to complete suspend until the process
sleeps (this wouldn't be true if you suspended the cpu with its
current stack - but if suspend is removing the non-boot cpus first
it must be doing so from the scheduler idle loop).

If you are doing suspend for aggressive power saving, then all the
processes (and processors) will already be idle. However you
probably wouldn't want the memory accesses to determine this on
a large NUMA system with 1024+ processors.

	David

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/27/2013 02:24 PM, David Laight wrote:
>>>>> It would also increase the latency of CPU-hotunplug operations.
>>>>
>>>> Is that a big deal?
>>>
>>> I thought that was the whole deal with this patchset - making cpu
>>> hotunplugs lighter and faster mostly for powersaving.  That said, just
>>> removing stop_machine call would be a pretty good deal and I don't
>>> know how meaningful reducing CPU hotunplug latency is.  Srivatsa?
>>>
>>
>> Keeping the hotunplug latency is important for suspend/resume, where
>> we take all non-boot CPUs in a loop. That's an interesting use-case
>> where intrusiveness doesn't matter much, but latency does. So yes,
>> making CPU hotplug faster is also one of the goals of this patchset.
> 
> If you are removing all but one of the cpu, the you only need
> one rcu cycle (remove everything from the list first).
> 

Hmm, yeah, but IIRC, back when we discussed this last time[1], we felt that
would make the code a little bit hard to understand. But I think we can
give it a shot to see how that goes and decide based on that. So thanks
for bringing that up again!

BTW, one thing I'd like to emphasize again here is that we will not use
the RCU-like concept to have 2 different masks - a stable online mask
and an actual online mask (this is one of the approaches that we had
discussed earlier[2]). The reason why we don't wanna go down that path is,
its hard to determine who can survive by just looking at the stable online
mask, and who needs to be aware of the actual online mask. That will
surely lead to more bugs and headache.

So the use of an RCU-like concept here would only be to ensure that all
preempt-disabled sections complete, and we can switch the synchronization
scheme to global rwlocks, like what we had proposed earlier[3]. So, that
still requires call-sites to be converted from preempt_disable() to
get/put_online_cpus_atomic().

I just wanted to clarify where exactly the RCU concept would fit in,
in the stop-machine() replacement scheme...


> I'd also guess that you can't suspend a cpu until you can sleep
> the process that is running on it - so if a process has pre-emption
> disabled you aren't going to complete suspend until the process
> sleeps (this wouldn't be true if you suspended the cpu with its
> current stack - but if suspend is removing the non-boot cpus first
> it must be doing so from the scheduler idle loop).
> 
> If you are doing suspend for aggressive power saving, then all the
> processes (and processors) will already be idle. However you
> probably wouldn't want the memory accesses to determine this on
> a large NUMA system with 1024+ processors.
> 

References:

[1]. http://lkml.indiana.edu/hypermail/linux/kernel/1212.2/01979.html
[2]. http://thread.gmane.org/gmane.linux.kernel/1405145/focus=29336
[3]. http://thread.gmane.org/gmane.linux.documentation/9520/focus=1443258
     http://thread.gmane.org/gmane.linux.power-management.general/29464/focus=1407948

Regards,
Srivatsa S. Bhat

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 03:29:40PM +0100, David Laight wrote:
> > Once stop_machine() is gone from the CPU offline path, we won't be able
> > to depend on disabling preemption to prevent CPUs from going offline
> > from under us.
> 
> Could you use an rcu-like sequence so that disabling pre-emption
> would be enough?
> 
> Something like rebuilding the cpu list, then forcing yourself
> to run on all the cpu.
> 
> That would be far less intrusive.

Maybe...  I must defer to Srivatsa on this question.

							Thanx, Paul

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 07:59 PM, David Laight wrote:
>> Once stop_machine() is gone from the CPU offline path, we won't be able
>> to depend on disabling preemption to prevent CPUs from going offline
>> from under us.
> 
> Could you use an rcu-like sequence so that disabling pre-emption
> would be enough?
> 
> Something like rebuilding the cpu list, then forcing yourself
> to run on all the cpu.
> 

Certainly, and we had debated schemes similar to that (including
schemes invoking synchronize_sched() itself) in earlier discussions.
(But IIRC even those schemes required converting call-sites from
preempt_disable() to get/put_online_cpus_atomic(), to properly
synchronize).

> That would be far less intrusive.
> 

But that would increase the latency of hotplug operations like Paul
pointed out, and that in turn is not good for use-cases such as
suspend/resume, where we take all non-boot CPUs offline in a loop.
(That would mean executing the above logic num_online_cpus() times!).

So we started hunting for ideas that can make the hotplug writer
side not only less intrusive, but also remain fast. That's how
the later designs evolved.
 
Regards,
Srivatsa S. Bhat

Re: [PATCH v2 15/45] rcu: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 07:39:40PM +0530, Srivatsa S. Bhat wrote:
> On 06/26/2013 03:30 AM, Paul E. McKenney wrote:
> > On Wed, Jun 26, 2013 at 01:57:55AM +0530, Srivatsa S. Bhat wrote:
> >> Once stop_machine() is gone from the CPU offline path, we won't be able
> >> to depend on disabling preemption to prevent CPUs from going offline
> >> from under us.
> >>
> >> In RCU code, rcu_implicit_dynticks_qs() checks if a CPU is offline,
> >> while being protected by a spinlock. Use the get/put_online_cpus_atomic()
> >> APIs to prevent CPUs from going offline, while invoking from atomic context.
> > 
> > I am not completely sure that this is needed.  Here is a (quite possibly
> > flawed) argument for its not being needed:
> > 
> > o	rcu_gp_init() holds off CPU-hotplug operations during
> > 	grace-period initialization.  Therefore, RCU will avoid
> > 	looking for quiescent states from CPUs that were offline
> > 	(and thus in an extended quiescent state) at the beginning
> > 	of the grace period.
> > 
> > o	If force_qs_rnp() is looking for a quiescent state from
> > 	a given CPU, and if it senses that CPU as being offline,
> > 	then even without synchronization we know that the CPU
> > 	was offline some time during the current grace period.
> > 
> > 	After all, it was online at the beginning of the grace
> > 	period (otherwise, we would not be looking at it at all),
> > 	and our later sampling of its state must have therefore
> > 	happened after the start of the grace period.  Given that
> > 	the grace period has not yet ended, it also has to happened
> > 	before the end of the grace period.
> > 
> > o	Therefore, we should be able to sample the offline state
> > 	without synchronization.
> >
> 
> Thanks a lot for explaining the synchronization design in detail, Paul!
> I agree that get/put_online_cpus_atomic() is not necessary here.
> 
> Regarding the debug checks under CONFIG_DEBUG_HOTPLUG_CPU, to avoid
> false-positives, I'm thinking of introducing a few _nocheck() variants,
> on a case-by-case basis, like cpu_is_offline_nocheck() (useful here in RCU)
> and for_each_online_cpu_nocheck() (useful in percpu-counter code, as
> pointed out by Tejun Heo). These fine synchronization details are kinda
> hard to encapsulate in that debug logic, so we can use the _nocheck()
> variants here to avoid getting splats when running with DEBUG_HOTPLUG_CPU
> enabled.

Good point, and seems like a reasonable approach to me.

							Thanx, Paul

[PATCH v2 16/45] tick-broadcast: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/time/tick-broadcast.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index 9d96a54..1e4ac45 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -227,12 +227,14 @@ static void tick_do_broadcast(struct cpumask *mask)
  */
 static void tick_do_periodic_broadcast(void)
 {
+	get_online_cpus_atomic();
 	raw_spin_lock(&tick_broadcast_lock);
 
 	cpumask_and(tmpmask, cpu_online_mask, tick_broadcast_mask);
 	tick_do_broadcast(tmpmask);
 
 	raw_spin_unlock(&tick_broadcast_lock);
+	put_online_cpus_atomic();
 }
 
 /*
@@ -335,11 +337,13 @@ out:
  */
 void tick_broadcast_on_off(unsigned long reason, int *oncpu)
 {
+	get_online_cpus_atomic();
 	if (!cpumask_test_cpu(*oncpu, cpu_online_mask))
 		printk(KERN_ERR "tick-broadcast: ignoring broadcast for "
 		       "offline CPU #%d\n", *oncpu);
 	else
 		tick_do_broadcast_on_off(&reason);
+	put_online_cpus_atomic();
 }
 
 /*
@@ -505,6 +509,7 @@ static void tick_handle_oneshot_broadcast(struct clock_event_device *dev)
 	ktime_t now, next_event;
 	int cpu, next_cpu = 0;
 
+	get_online_cpus_atomic();
 	raw_spin_lock(&tick_broadcast_lock);
 again:
 	dev->next_event.tv64 = KTIME_MAX;
@@ -562,6 +567,7 @@ again:
 			goto again;
 	}
 	raw_spin_unlock(&tick_broadcast_lock);
+	put_online_cpus_atomic();
 }
 
 /*
@@ -756,6 +762,7 @@ void tick_broadcast_switch_to_oneshot(void)
 	struct clock_event_device *bc;
 	unsigned long flags;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 
 	tick_broadcast_device.mode = TICKDEV_MODE_ONESHOT;
@@ -764,6 +771,7 @@ void tick_broadcast_switch_to_oneshot(void)
 		tick_broadcast_setup_oneshot(bc);
 
 	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
+	put_online_cpus_atomic();
 }
 
 

[PATCH v2 17/45] time/clocksource: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/time/clocksource.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index e713ef7..c4bbc25 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -30,6 +30,7 @@
 #include <linux/sched.h> /* for spin_unlock_irq() using preempt_count() m68k */
 #include <linux/tick.h>
 #include <linux/kthread.h>
+#include <linux/cpu.h>
 
 #include "tick-internal.h"
 
@@ -252,6 +253,7 @@ static void clocksource_watchdog(unsigned long data)
 	int64_t wd_nsec, cs_nsec;
 	int next_cpu, reset_pending;
 
+	get_online_cpus_atomic();
 	spin_lock(&watchdog_lock);
 	if (!watchdog_running)
 		goto out;
@@ -329,6 +331,7 @@ static void clocksource_watchdog(unsigned long data)
 	add_timer_on(&watchdog_timer, next_cpu);
 out:
 	spin_unlock(&watchdog_lock);
+	put_online_cpus_atomic();
 }
 
 static inline void clocksource_start_watchdog(void)
@@ -367,6 +370,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
 {
 	unsigned long flags;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&watchdog_lock, flags);
 	if (cs->flags & CLOCK_SOURCE_MUST_VERIFY) {
 		/* cs is a clocksource to be watched. */
@@ -386,6 +390,7 @@ static void clocksource_enqueue_watchdog(struct clocksource *cs)
 	/* Check if the watchdog timer needs to be started. */
 	clocksource_start_watchdog();
 	spin_unlock_irqrestore(&watchdog_lock, flags);
+	put_online_cpus_atomic();
 }
 
 static void clocksource_dequeue_watchdog(struct clocksource *cs)

[PATCH v2 18/45] softirq: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/softirq.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 3d6833f..c289722 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -644,14 +644,17 @@ static void remote_softirq_receive(void *data)
 
 static int __try_remote_softirq(struct call_single_data *cp, int cpu, int softirq)
 {
+	get_online_cpus_atomic();
 	if (cpu_online(cpu)) {
 		cp->func = remote_softirq_receive;
 		cp->info = &softirq;
 		cp->flags = 0;
 
 		__smp_call_function_single(cpu, cp, 0);
+		put_online_cpus_atomic();
 		return 0;
 	}
+	put_online_cpus_atomic();
 	return 1;
 }
 #else /* CONFIG_USE_GENERIC_SMP_HELPERS */

[PATCH v2 19/45] irq: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/irq/manage.c |    7 +++++++
 kernel/irq/proc.c   |    3 +++
 2 files changed, 10 insertions(+)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index e16caa8..4d89f19 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -18,6 +18,7 @@
 #include <linux/sched.h>
 #include <linux/sched/rt.h>
 #include <linux/task_work.h>
+#include <linux/cpu.h>
 
 #include "internals.h"
 
@@ -202,9 +203,11 @@ int irq_set_affinity(unsigned int irq, const struct cpumask *mask)
 	if (!desc)
 		return -EINVAL;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&desc->lock, flags);
 	ret =  __irq_set_affinity_locked(irq_desc_get_irq_data(desc), mask);
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
+	put_online_cpus_atomic();
 	return ret;
 }
 
@@ -343,9 +346,11 @@ int irq_select_affinity_usr(unsigned int irq, struct cpumask *mask)
 	unsigned long flags;
 	int ret;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&desc->lock, flags);
 	ret = setup_affinity(irq, desc, mask);
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
+	put_online_cpus_atomic();
 	return ret;
 }
 
@@ -1128,7 +1133,9 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 		}
 
 		/* Set default affinity mask once everything is setup */
+		get_online_cpus_atomic();
 		setup_affinity(irq, desc, mask);
+		put_online_cpus_atomic();
 
 	} else if (new->flags & IRQF_TRIGGER_MASK) {
 		unsigned int nmsk = new->flags & IRQF_TRIGGER_MASK;
diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
index 19ed5c4..47f9a74 100644
--- a/kernel/irq/proc.c
+++ b/kernel/irq/proc.c
@@ -7,6 +7,7 @@
  */
 
 #include <linux/irq.h>
+#include <linux/cpu.h>
 #include <linux/gfp.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
@@ -441,6 +442,7 @@ int show_interrupts(struct seq_file *p, void *v)
 	if (!desc)
 		return 0;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&desc->lock, flags);
 	for_each_online_cpu(j)
 		any_count |= kstat_irqs_cpu(i, j);
@@ -477,6 +479,7 @@ int show_interrupts(struct seq_file *p, void *v)
 	seq_putc(p, '\n');
 out:
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
+	put_online_cpus_atomic();
 	return 0;
 }
 #endif

[PATCH v2 20/45] net: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 net/core/dev.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index fc1e289..90519e9 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3141,7 +3141,7 @@ int netif_rx(struct sk_buff *skb)
 		struct rps_dev_flow voidflow, *rflow = &voidflow;
 		int cpu;
 
-		preempt_disable();
+		get_online_cpus_atomic();
 		rcu_read_lock();
 
 		cpu = get_rps_cpu(skb->dev, skb, &rflow);
@@ -3151,7 +3151,7 @@ int netif_rx(struct sk_buff *skb)
 		ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
 
 		rcu_read_unlock();
-		preempt_enable();
+		put_online_cpus_atomic();
 	} else
 #endif
 	{
@@ -3570,6 +3570,7 @@ int netif_receive_skb(struct sk_buff *skb)
 		struct rps_dev_flow voidflow, *rflow = &voidflow;
 		int cpu, ret;
 
+		get_online_cpus_atomic();
 		rcu_read_lock();
 
 		cpu = get_rps_cpu(skb->dev, skb, &rflow);
@@ -3577,9 +3578,11 @@ int netif_receive_skb(struct sk_buff *skb)
 		if (cpu >= 0) {
 			ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
 			rcu_read_unlock();
+			put_online_cpus_atomic();
 			return ret;
 		}
 		rcu_read_unlock();
+		put_online_cpus_atomic();
 	}
 #endif
 	return __netif_receive_skb(skb);
@@ -3957,6 +3960,7 @@ static void net_rps_action_and_irq_enable(struct softnet_data *sd)
 		local_irq_enable();
 
 		/* Send pending IPI's to kick RPS processing on remote cpus. */
+		get_online_cpus_atomic();
 		while (remsd) {
 			struct softnet_data *next = remsd->rps_ipi_next;
 
@@ -3965,6 +3969,7 @@ static void net_rps_action_and_irq_enable(struct softnet_data *sd)
 							   &remsd->csd, 0);
 			remsd = next;
 		}
+		put_online_cpus_atomic();
 	} else
 #endif
 		local_irq_enable();

[PATCH v2 21/45] block: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 block/blk-softirq.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-softirq.c b/block/blk-softirq.c
index 467c8de..bbab3d3 100644
--- a/block/blk-softirq.c
+++ b/block/blk-softirq.c
@@ -58,6 +58,7 @@ static void trigger_softirq(void *data)
  */
 static int raise_blk_irq(int cpu, struct request *rq)
 {
+	get_online_cpus_atomic();
 	if (cpu_online(cpu)) {
 		struct call_single_data *data = &rq->csd;
 
@@ -66,8 +67,10 @@ static int raise_blk_irq(int cpu, struct request *rq)
 		data->flags = 0;
 
 		__smp_call_function_single(cpu, data, 0);
+		put_online_cpus_atomic();
 		return 0;
 	}
+	put_online_cpus_atomic();
 
 	return 1;
 }

[PATCH v2 22/45] percpu_counter: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 lib/percpu_counter.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index ba6085d..f5e718d 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -98,6 +98,15 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
 	s64 ret;
 	int cpu;
 
+	/*
+	 * The calls to get/put_online_cpus_atomic() is strictly not
+	 * necessary, since CPU hotplug is explicitly handled via the
+	 * hotplug callback which synchronizes through fbc->lock.
+	 * But we add them here anyway to make it easier for the debug
+	 * code under CONFIG_DEBUG_HOTPLUG_CPU to validate the correctness
+	 * of hotplug synchronization.
+	 */
+	get_online_cpus_atomic();
 	raw_spin_lock(&fbc->lock);
 	ret = fbc->count;
 	for_each_online_cpu(cpu) {
@@ -105,6 +114,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
 		ret += *pcount;
 	}
 	raw_spin_unlock(&fbc->lock);
+	put_online_cpus_atomic();
 	return ret;
 }
 EXPORT_SYMBOL(__percpu_counter_sum);

[PATCH v2 23/45] infiniband: ehca: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Cc: Christoph Raisch <raisch@de.ibm.com>
Cc: Roland Dreier <roland@kernel.org>
Cc: Sean Hefty <sean.hefty@intel.com>
Cc: Hal Rosenstock <hal.rosenstock@gmail.com>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 drivers/infiniband/hw/ehca/ehca_irq.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/infiniband/hw/ehca/ehca_irq.c b/drivers/infiniband/hw/ehca/ehca_irq.c
index 8615d7c..ace901e 100644
--- a/drivers/infiniband/hw/ehca/ehca_irq.c
+++ b/drivers/infiniband/hw/ehca/ehca_irq.c
@@ -43,6 +43,7 @@
 
 #include <linux/slab.h>
 #include <linux/smpboot.h>
+#include <linux/cpu.h>
 
 #include "ehca_classes.h"
 #include "ehca_irq.h"
@@ -703,6 +704,7 @@ static void queue_comp_task(struct ehca_cq *__cq)
 	int cq_jobs;
 	unsigned long flags;
 
+	get_online_cpus_atomic();
 	cpu_id = find_next_online_cpu(pool);
 	BUG_ON(!cpu_online(cpu_id));
 
@@ -720,6 +722,7 @@ static void queue_comp_task(struct ehca_cq *__cq)
 		BUG_ON(!cct || !thread);
 	}
 	__queue_comp_task(__cq, cct, thread);
+	put_online_cpus_atomic();
 }
 
 static void run_comp_task(struct ehca_cpu_comp_task *cct)
@@ -759,6 +762,7 @@ static void comp_task_park(unsigned int cpu)
 	list_splice_init(&cct->cq_list, &list);
 	spin_unlock_irq(&cct->task_lock);
 
+	get_online_cpus_atomic();
 	cpu = find_next_online_cpu(pool);
 	target = per_cpu_ptr(pool->cpu_comp_tasks, cpu);
 	thread = *per_cpu_ptr(pool->cpu_comp_threads, cpu);
@@ -768,6 +772,7 @@ static void comp_task_park(unsigned int cpu)
 		__queue_comp_task(cq, target, thread);
 	}
 	spin_unlock_irq(&target->task_lock);
+	put_online_cpus_atomic();
 }
 
 static void comp_task_stop(unsigned int cpu, bool online)

[PATCH v2 24/45] [SCSI] fcoe: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Robert Love <robert.w.love@intel.com>
Cc: "James E.J. Bottomley" <JBottomley@parallels.com>
Cc: devel@open-fcoe.org
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 drivers/scsi/fcoe/fcoe.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/fcoe/fcoe.c b/drivers/scsi/fcoe/fcoe.c
index 292b24f..a107d3c 100644
--- a/drivers/scsi/fcoe/fcoe.c
+++ b/drivers/scsi/fcoe/fcoe.c
@@ -1484,6 +1484,7 @@ static int fcoe_rcv(struct sk_buff *skb, struct net_device *netdev,
 	 * was originated, otherwise select cpu using rx exchange id
 	 * or fcoe_select_cpu().
 	 */
+	get_online_cpus_atomic();
 	if (ntoh24(fh->fh_f_ctl) & FC_FC_EX_CTX)
 		cpu = ntohs(fh->fh_ox_id) & fc_cpu_mask;
 	else {
@@ -1493,8 +1494,10 @@ static int fcoe_rcv(struct sk_buff *skb, struct net_device *netdev,
 			cpu = ntohs(fh->fh_rx_id) & fc_cpu_mask;
 	}
 
-	if (cpu >= nr_cpu_ids)
+	if (cpu >= nr_cpu_ids) {
+		put_online_cpus_atomic();
 		goto err;
+	}
 
 	fps = &per_cpu(fcoe_percpu, cpu);
 	spin_lock(&fps->fcoe_rx_list.lock);
@@ -1514,6 +1517,7 @@ static int fcoe_rcv(struct sk_buff *skb, struct net_device *netdev,
 		spin_lock(&fps->fcoe_rx_list.lock);
 		if (!fps->thread) {
 			spin_unlock(&fps->fcoe_rx_list.lock);
+			put_online_cpus_atomic();
 			goto err;
 		}
 	}
@@ -1535,6 +1539,7 @@ static int fcoe_rcv(struct sk_buff *skb, struct net_device *netdev,
 	if (fps->thread->state == TASK_INTERRUPTIBLE)
 		wake_up_process(fps->thread);
 	spin_unlock(&fps->fcoe_rx_list.lock);
+	put_online_cpus_atomic();
 
 	return 0;
 err:

[PATCH v2 25/45] staging/octeon: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: devel@driverdev.osuosl.org
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 drivers/staging/octeon/ethernet-rx.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/staging/octeon/ethernet-rx.c b/drivers/staging/octeon/ethernet-rx.c
index 34afc16..8588b4d 100644
--- a/drivers/staging/octeon/ethernet-rx.c
+++ b/drivers/staging/octeon/ethernet-rx.c
@@ -36,6 +36,7 @@
 #include <linux/prefetch.h>
 #include <linux/ratelimit.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/interrupt.h>
 #include <net/dst.h>
 #ifdef CONFIG_XFRM
@@ -97,6 +98,7 @@ static void cvm_oct_enable_one_cpu(void)
 		return;
 
 	/* ... if a CPU is available, Turn on NAPI polling for that CPU.  */
+	get_online_cpus_atomic();
 	for_each_online_cpu(cpu) {
 		if (!cpu_test_and_set(cpu, core_state.cpu_state)) {
 			v = smp_call_function_single(cpu, cvm_oct_enable_napi,
@@ -106,6 +108,7 @@ static void cvm_oct_enable_one_cpu(void)
 			break;
 		}
 	}
+	put_online_cpus_atomic();
 }
 
 static void cvm_oct_no_more_work(void)

Re: [PATCH v2 25/45] staging/octeon: Use get/put_online_cpus_atomic() to prevent CPU offline

[Greg Kroah-Hartman]

On Wed, Jun 26, 2013 at 02:00:04AM +0530, Srivatsa S. Bhat wrote:
> Once stop_machine() is gone from the CPU offline path, we won't be able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.
> 
> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
> offline, while invoking from atomic context.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: devel@driverdev.osuosl.org
> Acked-by: David Daney <david.daney@cavium.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

[PATCH v2 26/45] x86: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Tony Luck <tony.luck@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: linux-edac@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kernel/apic/io_apic.c           |   21 ++++++++++++++++++---
 arch/x86/kernel/cpu/mcheck/therm_throt.c |    4 ++--
 arch/x86/mm/tlb.c                        |   14 +++++++-------
 3 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 9ed796c..4c71c1e 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -25,6 +25,7 @@
 #include <linux/init.h>
 #include <linux/delay.h>
 #include <linux/sched.h>
+#include <linux/cpu.h>
 #include <linux/pci.h>
 #include <linux/mc146818rtc.h>
 #include <linux/compiler.h>
@@ -1169,9 +1170,11 @@ int assign_irq_vector(int irq, struct irq_cfg *cfg, const struct cpumask *mask)
 	int err;
 	unsigned long flags;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&vector_lock, flags);
 	err = __assign_irq_vector(irq, cfg, mask);
 	raw_spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	return err;
 }
 
@@ -1757,13 +1760,13 @@ __apicdebuginit(void) print_local_APICs(int maxcpu)
 	if (!maxcpu)
 		return;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	for_each_online_cpu(cpu) {
 		if (cpu >= maxcpu)
 			break;
 		smp_call_function_single(cpu, print_local_APIC, NULL, 1);
 	}
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 __apicdebuginit(void) print_PIC(void)
@@ -2153,10 +2156,12 @@ static int ioapic_retrigger_irq(struct irq_data *data)
 	unsigned long flags;
 	int cpu;
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&vector_lock, flags);
 	cpu = cpumask_first_and(cfg->domain, cpu_online_mask);
 	apic->send_IPI_mask(cpumask_of(cpu), cfg->vector);
 	raw_spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 
 	return 1;
 }
@@ -2175,6 +2180,7 @@ void send_cleanup_vector(struct irq_cfg *cfg)
 {
 	cpumask_var_t cleanup_mask;
 
+	get_online_cpus_atomic();
 	if (unlikely(!alloc_cpumask_var(&cleanup_mask, GFP_ATOMIC))) {
 		unsigned int i;
 		for_each_cpu_and(i, cfg->old_domain, cpu_online_mask)
@@ -2185,6 +2191,7 @@ void send_cleanup_vector(struct irq_cfg *cfg)
 		free_cpumask_var(cleanup_mask);
 	}
 	cfg->move_in_progress = 0;
+	put_online_cpus_atomic();
 }
 
 asmlinkage void smp_irq_move_cleanup_interrupt(void)
@@ -2939,11 +2946,13 @@ unsigned int __create_irqs(unsigned int from, unsigned int count, int node)
 			goto out_irqs;
 	}
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&vector_lock, flags);
 	for (i = 0; i < count; i++)
 		if (__assign_irq_vector(irq + i, cfg[i], apic->target_cpus()))
 			goto out_vecs;
 	raw_spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 
 	for (i = 0; i < count; i++) {
 		irq_set_chip_data(irq + i, cfg[i]);
@@ -2957,6 +2966,7 @@ out_vecs:
 	for (i--; i >= 0; i--)
 		__clear_irq_vector(irq + i, cfg[i]);
 	raw_spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 out_irqs:
 	for (i = 0; i < count; i++)
 		free_irq_at(irq + i, cfg[i]);
@@ -2994,9 +3004,11 @@ void destroy_irq(unsigned int irq)
 
 	free_remapped_irq(irq);
 
+	get_online_cpus_atomic();
 	raw_spin_lock_irqsave(&vector_lock, flags);
 	__clear_irq_vector(irq, cfg);
 	raw_spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	free_irq_at(irq, cfg);
 }
 
@@ -3365,8 +3377,11 @@ io_apic_setup_irq_pin(unsigned int irq, int node, struct io_apic_irq_attr *attr)
 	if (!cfg)
 		return -EINVAL;
 	ret = __add_pin_to_irq_node(cfg, node, attr->ioapic, attr->ioapic_pin);
-	if (!ret)
+	if (!ret) {
+		get_online_cpus_atomic();
 		setup_ioapic_irq(irq, cfg, attr);
+		put_online_cpus_atomic();
+	}
 	return ret;
 }
 
diff --git a/arch/x86/kernel/cpu/mcheck/therm_throt.c b/arch/x86/kernel/cpu/mcheck/therm_throt.c
index 2f3a799..3eea984 100644
--- a/arch/x86/kernel/cpu/mcheck/therm_throt.c
+++ b/arch/x86/kernel/cpu/mcheck/therm_throt.c
@@ -83,13 +83,13 @@ static ssize_t therm_throt_device_show_##event##_##name(		\
 	unsigned int cpu = dev->id;					\
 	ssize_t ret;							\
 									\
-	preempt_disable();	/* CPU hotplug */			\
+	get_online_cpus_atomic();	/* CPU hotplug */		\
 	if (cpu_online(cpu)) {						\
 		ret = sprintf(buf, "%lu\n",				\
 			      per_cpu(thermal_state, cpu).event.name);	\
 	} else								\
 		ret = 0;						\
-	preempt_enable();						\
+	put_online_cpus_atomic();					\
 									\
 	return ret;							\
 }
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 282375f..8126374 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -147,12 +147,12 @@ void flush_tlb_current_task(void)
 {
 	struct mm_struct *mm = current->mm;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	local_flush_tlb();
 	if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids)
 		flush_tlb_others(mm_cpumask(mm), mm, 0UL, TLB_FLUSH_ALL);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /*
@@ -187,7 +187,7 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 	unsigned long addr;
 	unsigned act_entries, tlb_entries = 0;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	if (current->active_mm != mm)
 		goto flush_all;
 
@@ -225,21 +225,21 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
 		if (cpumask_any_but(mm_cpumask(mm),
 				smp_processor_id()) < nr_cpu_ids)
 			flush_tlb_others(mm_cpumask(mm), mm, start, end);
-		preempt_enable();
+		put_online_cpus_atomic();
 		return;
 	}
 
 flush_all:
 	if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids)
 		flush_tlb_others(mm_cpumask(mm), mm, 0UL, TLB_FLUSH_ALL);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
 {
 	struct mm_struct *mm = vma->vm_mm;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if (current->active_mm == mm) {
 		if (current->mm)
@@ -251,7 +251,7 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long start)
 	if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids)
 		flush_tlb_others(mm_cpumask(mm), mm, start, 0UL);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 static void do_flush_tlb_all(void *info)

[PATCH v2 27/45] perf/x86: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

The CPU_DYING notifier modifies the per-cpu pointer pmu->box, and this can
race with functions such as uncore_pmu_to_box() and uncore_pci_remove() when
we remove stop_machine() from the CPU offline path. So protect them using
get/put_online_cpus_atomic().

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kernel/cpu/perf_event_intel_uncore.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_uncore.c b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
index 9dd9975..7c2a064 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_uncore.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_uncore.c
@@ -1,3 +1,4 @@
+#include <linux/cpu.h>
 #include "perf_event_intel_uncore.h"
 
 static struct intel_uncore_type *empty_uncore[] = { NULL, };
@@ -2630,6 +2631,7 @@ uncore_pmu_to_box(struct intel_uncore_pmu *pmu, int cpu)
 	if (box)
 		return box;
 
+	get_online_cpus_atomic();
 	raw_spin_lock(&uncore_box_lock);
 	list_for_each_entry(box, &pmu->box_list, list) {
 		if (box->phys_id == topology_physical_package_id(cpu)) {
@@ -2639,6 +2641,7 @@ uncore_pmu_to_box(struct intel_uncore_pmu *pmu, int cpu)
 		}
 	}
 	raw_spin_unlock(&uncore_box_lock);
+	put_online_cpus_atomic();
 
 	return *per_cpu_ptr(pmu->box, cpu);
 }
@@ -3229,6 +3232,7 @@ static void uncore_pci_remove(struct pci_dev *pdev)
 	list_del(&box->list);
 	raw_spin_unlock(&uncore_box_lock);
 
+	get_online_cpus_atomic();
 	for_each_possible_cpu(cpu) {
 		if (*per_cpu_ptr(pmu->box, cpu) == box) {
 			*per_cpu_ptr(pmu->box, cpu) = NULL;
@@ -3237,6 +3241,8 @@ static void uncore_pci_remove(struct pci_dev *pdev)
 	}
 
 	WARN_ON_ONCE(atomic_read(&box->refcnt) != 1);
+	put_online_cpus_atomic();
+
 	kfree(box);
 }
 

[PATCH v2 28/45] KVM: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Gleb Natapov <gleb@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 virt/kvm/kvm_main.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 302681c..5bbfa30 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -174,7 +174,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 
 	zalloc_cpumask_var(&cpus, GFP_ATOMIC);
 
-	me = get_cpu();
+	me = get_online_cpus_atomic();
 	kvm_for_each_vcpu(i, vcpu, kvm) {
 		kvm_make_request(req, vcpu);
 		cpu = vcpu->cpu;
@@ -192,7 +192,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
 		smp_call_function_many(cpus, ack_flush, NULL, 1);
 	else
 		called = false;
-	put_cpu();
+	put_online_cpus_atomic();
 	free_cpumask_var(cpus);
 	return called;
 }
@@ -1707,11 +1707,11 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
 		++vcpu->stat.halt_wakeup;
 	}
 
-	me = get_cpu();
+	me = get_online_cpus_atomic();
 	if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
 		if (kvm_arch_vcpu_should_kick(vcpu))
 			smp_send_reschedule(cpu);
-	put_cpu();
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_kick);
 #endif /* !CONFIG_S390 */

Re: [PATCH v2 28/45] KVM: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paolo Bonzini]

Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
> Once stop_machine() is gone from the CPU offline path, we won't be able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.
> 
> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
> offline, while invoking from atomic context.
> 
> Cc: Gleb Natapov <gleb@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: kvm@vger.kernel.org
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
> ---
> 
>  virt/kvm/kvm_main.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 302681c..5bbfa30 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -174,7 +174,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
>  
>  	zalloc_cpumask_var(&cpus, GFP_ATOMIC);
>  
> -	me = get_cpu();
> +	me = get_online_cpus_atomic();
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		kvm_make_request(req, vcpu);
>  		cpu = vcpu->cpu;
> @@ -192,7 +192,7 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
>  		smp_call_function_many(cpus, ack_flush, NULL, 1);
>  	else
>  		called = false;
> -	put_cpu();
> +	put_online_cpus_atomic();
>  	free_cpumask_var(cpus);
>  	return called;
>  }
> @@ -1707,11 +1707,11 @@ void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
>  		++vcpu->stat.halt_wakeup;
>  	}
>  
> -	me = get_cpu();
> +	me = get_online_cpus_atomic();
>  	if (cpu != me && (unsigned)cpu < nr_cpu_ids && cpu_online(cpu))
>  		if (kvm_arch_vcpu_should_kick(vcpu))
>  			smp_send_reschedule(cpu);
> -	put_cpu();
> +	put_online_cpus_atomic();
>  }
>  EXPORT_SYMBOL_GPL(kvm_vcpu_kick);
>  #endif /* !CONFIG_S390 */
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-pm" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>

[PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Gleb Natapov <gleb@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: kvm@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kvm/vmx.c |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 260a919..4e1e966 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -26,6 +26,7 @@
 #include <linux/mm.h>
 #include <linux/highmem.h>
 #include <linux/sched.h>
+#include <linux/cpu.h>
 #include <linux/moduleparam.h>
 #include <linux/mod_devicetable.h>
 #include <linux/ftrace_event.h>
@@ -7164,12 +7165,12 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
 	if (!vmm_exclusive)
 		kvm_cpu_vmxoff();
 
-	cpu = get_cpu();
+	cpu = get_online_cpus_atomic();
 	vmx_vcpu_load(&vmx->vcpu, cpu);
 	vmx->vcpu.cpu = cpu;
 	err = vmx_vcpu_setup(vmx);
 	vmx_vcpu_put(&vmx->vcpu);
-	put_cpu();
+	put_online_cpus_atomic();
 	if (err)
 		goto free_vmcs;
 	if (vm_need_virtualize_apic_accesses(kvm)) {
@@ -7706,12 +7707,12 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
 
 	vmx->nested.vmcs01_tsc_offset = vmcs_read64(TSC_OFFSET);
 
-	cpu = get_cpu();
+	cpu = get_online_cpus_atomic();
 	vmx->loaded_vmcs = vmcs02;
 	vmx_vcpu_put(vcpu);
 	vmx_vcpu_load(vcpu, cpu);
 	vcpu->cpu = cpu;
-	put_cpu();
+	put_online_cpus_atomic();
 
 	vmx_segment_cache_clear(vmx);
 
@@ -8023,12 +8024,12 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu)
 	leave_guest_mode(vcpu);
 	prepare_vmcs12(vcpu, vmcs12);
 
-	cpu = get_cpu();
+	cpu = get_online_cpus_atomic();
 	vmx->loaded_vmcs = &vmx->vmcs01;
 	vmx_vcpu_put(vcpu);
 	vmx_vcpu_load(vcpu, cpu);
 	vcpu->cpu = cpu;
-	put_cpu();
+	put_online_cpus_atomic();
 
 	vmx_segment_cache_clear(vmx);
 

Re: [PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paolo Bonzini]

Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
> -	cpu = get_cpu();
> +	cpu = get_online_cpus_atomic();
>  	vmx_vcpu_load(&vmx->vcpu, cpu);
>  	vmx->vcpu.cpu = cpu;
>  	err = vmx_vcpu_setup(vmx);
>  	vmx_vcpu_put(&vmx->vcpu);
> -	put_cpu();
> +	put_online_cpus_atomic();

The new API has a weird name.  Why are you adding new functions instead
of just modifying get/put_cpu?

Paolo

Re: [PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 01:16 PM, Paolo Bonzini wrote:
> Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
>> -	cpu = get_cpu();
>> +	cpu = get_online_cpus_atomic();
>>  	vmx_vcpu_load(&vmx->vcpu, cpu);
>>  	vmx->vcpu.cpu = cpu;
>>  	err = vmx_vcpu_setup(vmx);
>>  	vmx_vcpu_put(&vmx->vcpu);
>> -	put_cpu();
>> +	put_online_cpus_atomic();
> 
> The new API has a weird name.  Why are you adding new functions instead
> of just modifying get/put_cpu?
> 

Because the purpose of those two functions are distinctly different
from each other.

get/put_cpu() is used to disable preemption on the local CPU. (Which
also disables offlining the local CPU during that critical section).

What this patchset deals with is synchronizing with offline of *any*
CPU. Typically, we use get_online_cpus()/put_online_cpus() for that
purpose. But they can't be used in atomic context, because they take
mutex locks and hence can sleep.

So the code that executes in atomic context and which wants to prevent
*any* CPU from going offline, used to disable preemption around its
critical section. Disabling preemption prevents stop_machine(), and
CPU offline (of *any* CPU) was done via stop_machine(). So disabling
preemption disabled any CPU from going offline, as a *side-effect*.

And this patchset prepares the ground for getting rid of stop_machine()
in the CPU offline path. Which means, disabling preemption only prevents
the *local* CPU from going offline. So if code in atomic context wants
to prevent any CPU from going offline, we need a new set of APIs, like
get/put_online_cpus(), but which can be invoked from atomic context.
That's why I named it as get/put_online_cpus_atomic().

One of the key points here is that we want to preserve get/put_cpu()
as it is, since its purpose is different - disable preemption and
offline of the local CPU. There is no reason to change that API, its
useful as it is.

Regards,
Srivatsa S. Bhat

Re: [PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paolo Bonzini]

Il 26/06/2013 10:06, Srivatsa S. Bhat ha scritto:
> On 06/26/2013 01:16 PM, Paolo Bonzini wrote:
>> Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
>>> -	cpu = get_cpu();
>>> +	cpu = get_online_cpus_atomic();
>>>  	vmx_vcpu_load(&vmx->vcpu, cpu);
>>>  	vmx->vcpu.cpu = cpu;
>>>  	err = vmx_vcpu_setup(vmx);
>>>  	vmx_vcpu_put(&vmx->vcpu);
>>> -	put_cpu();
>>> +	put_online_cpus_atomic();
>>
>> The new API has a weird name.  Why are you adding new functions instead
>> of just modifying get/put_cpu?
>>
> 
> Because the purpose of those two functions are distinctly different
> from each other.
> 
> get/put_cpu() is used to disable preemption on the local CPU. (Which
> also disables offlining the local CPU during that critical section).

Ok, then I understood correctly... and I acked the other KVM patch.

However, keeping the code on the local CPU is exactly the point of this
particular use of get_cpu()/put_cpu().  Why does it need to synchronize
with offlining of other CPUs?

Paolo

> What this patchset deals with is synchronizing with offline of *any*
> CPU. Typically, we use get_online_cpus()/put_online_cpus() for that
> purpose. But they can't be used in atomic context, because they take
> mutex locks and hence can sleep.
> 
> So the code that executes in atomic context and which wants to prevent
> *any* CPU from going offline, used to disable preemption around its
> critical section. Disabling preemption prevents stop_machine(), and
> CPU offline (of *any* CPU) was done via stop_machine(). So disabling
> preemption disabled any CPU from going offline, as a *side-effect*.
> 
> And this patchset prepares the ground for getting rid of stop_machine()
> in the CPU offline path. Which means, disabling preemption only prevents
> the *local* CPU from going offline. So if code in atomic context wants
> to prevent any CPU from going offline, we need a new set of APIs, like
> get/put_online_cpus(), but which can be invoked from atomic context.
> That's why I named it as get/put_online_cpus_atomic().
> 
> One of the key points here is that we want to preserve get/put_cpu()
> as it is, since its purpose is different - disable preemption and
> offline of the local CPU. There is no reason to change that API, its
> useful as it is.

Re: [PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 01:53 PM, Paolo Bonzini wrote:
> Il 26/06/2013 10:06, Srivatsa S. Bhat ha scritto:
>> On 06/26/2013 01:16 PM, Paolo Bonzini wrote:
>>> Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
>>>> -	cpu = get_cpu();
>>>> +	cpu = get_online_cpus_atomic();
>>>>  	vmx_vcpu_load(&vmx->vcpu, cpu);
>>>>  	vmx->vcpu.cpu = cpu;
>>>>  	err = vmx_vcpu_setup(vmx);
>>>>  	vmx_vcpu_put(&vmx->vcpu);
>>>> -	put_cpu();
>>>> +	put_online_cpus_atomic();
>>>
>>> The new API has a weird name.  Why are you adding new functions instead
>>> of just modifying get/put_cpu?
>>>
>>
>> Because the purpose of those two functions are distinctly different
>> from each other.
>>
>> get/put_cpu() is used to disable preemption on the local CPU. (Which
>> also disables offlining the local CPU during that critical section).
> 
> Ok, then I understood correctly... and I acked the other KVM patch.
>

Thank you!
 
> However, keeping the code on the local CPU is exactly the point of this
> particular use of get_cpu()/put_cpu().  Why does it need to synchronize
> with offlining of other CPUs?
> 

Now that I looked at it again, I think you are right, get/put_cpu() is
good enough here.

But let me explain why I initially thought we needed full synchronization
with CPU offline. In short, I wanted to synchronize the calls to
__loaded_vmcs_clear(). We have the scenario shown below:

CPU offline:
	CPU_DYING:
		hardware_disable();
		->vmclear_local_loaded_vmcss();
		  ->__loaded_vmcs_clear(v);



And vmx_vcpu_load() (among others) can do:
       vmx_vcpu_load();
       -> loaded_vmcs_clear();
          -> __loaded_vmcs_clear();


So I wanted to avoid this race-condition and hence wrapped the code with
get/put_online_cpus_atomic().

But the point I missed earlier is that loaded_vmcs_clear() calls
__loaded_vmcs_clear() using smp_call_function_single(), which itself
synchronizes properly with CPU hotplug. So there is no need to add full
hotplug synchronization in the vmx code, as you noted above.

So, please ignore this patch, and sorry for the noise!

Regards,
Srivatsa S. Bhat

Re: [PATCH v2 29/45] kvm/vmx: Use get/put_online_cpus_atomic() to prevent CPU offline

[Paolo Bonzini]

Il 26/06/2013 10:41, Srivatsa S. Bhat ha scritto:
> On 06/26/2013 01:53 PM, Paolo Bonzini wrote:
>> Il 26/06/2013 10:06, Srivatsa S. Bhat ha scritto:
>>> On 06/26/2013 01:16 PM, Paolo Bonzini wrote:
>>>> Il 25/06/2013 22:30, Srivatsa S. Bhat ha scritto:
>>>>> -	cpu = get_cpu();
>>>>> +	cpu = get_online_cpus_atomic();
>>>>>  	vmx_vcpu_load(&vmx->vcpu, cpu);
>>>>>  	vmx->vcpu.cpu = cpu;
>>>>>  	err = vmx_vcpu_setup(vmx);
>>>>>  	vmx_vcpu_put(&vmx->vcpu);
>>>>> -	put_cpu();
>>>>> +	put_online_cpus_atomic();
>>>>
>>>> The new API has a weird name.  Why are you adding new functions instead
>>>> of just modifying get/put_cpu?
>>>>
>>>
>>> Because the purpose of those two functions are distinctly different
>>> from each other.
>>>
>>> get/put_cpu() is used to disable preemption on the local CPU. (Which
>>> also disables offlining the local CPU during that critical section).
>>
>> Ok, then I understood correctly... and I acked the other KVM patch.
>>
> 
> Thank you!
>  
>> However, keeping the code on the local CPU is exactly the point of this
>> particular use of get_cpu()/put_cpu().  Why does it need to synchronize
>> with offlining of other CPUs?
> 
> Now that I looked at it again, I think you are right, get/put_cpu() is
> good enough here.
> 
> But let me explain why I initially thought we needed full synchronization
> with CPU offline. In short, I wanted to synchronize the calls to
> __loaded_vmcs_clear(). We have the scenario shown below:
> 
> CPU offline:
> 	CPU_DYING:
> 		hardware_disable();
> 		->vmclear_local_loaded_vmcss();
> 		  ->__loaded_vmcs_clear(v);
> 
> 
> 
> And vmx_vcpu_load() (among others) can do:
>        vmx_vcpu_load();
>        -> loaded_vmcs_clear();
>           -> __loaded_vmcs_clear();
> 
> 
> So I wanted to avoid this race-condition and hence wrapped the code with
> get/put_online_cpus_atomic().
> 
> But the point I missed earlier is that loaded_vmcs_clear() calls
> __loaded_vmcs_clear() using smp_call_function_single(), which itself
> synchronizes properly with CPU hotplug. So there is no need to add full
> hotplug synchronization in the vmx code, as you noted above.

Makes sense, and I see now that it's patch 9 in this series.

In general, I would rather add an extra get_online_cpus_atomic pair
where it it actually needed (i.e. closer to where cpu_online is actually
used), and leave get_cpu/put_cpu as is in the caller... which is exactly
what happens in this case, since "where it is actually needed" is "in
smp_call_function_single()".

> So, please ignore this patch, and sorry for the noise!

No problem, thanks for the heads-up.

Paolo

[PATCH v2 30/45] x86/xen: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: xen-devel@lists.xensource.com
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/xen/mmu.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index fdc3ba2..3229c4f 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -39,6 +39,7 @@
  * Jeremy Fitzhardinge <jeremy@xensource.com>, XenSource Inc, 2007
  */
 #include <linux/sched.h>
+#include <linux/cpu.h>
 #include <linux/highmem.h>
 #include <linux/debugfs.h>
 #include <linux/bug.h>
@@ -1163,9 +1164,13 @@ static void xen_drop_mm_ref(struct mm_struct *mm)
  */
 static void xen_exit_mmap(struct mm_struct *mm)
 {
-	get_cpu();		/* make sure we don't move around */
+	/*
+	 * Make sure we don't move around, and also prevent CPUs from
+	 * going offline.
+	 */
+	get_online_cpus_atomic();
 	xen_drop_mm_ref(mm);
-	put_cpu();
+	put_online_cpus_atomic();
 
 	spin_lock(&mm->page_table_lock);
 

[PATCH v2 31/45] alpha/smp: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-alpha@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/alpha/kernel/smp.c |   19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 7b60834..e147268 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -497,7 +497,6 @@ smp_cpus_done(unsigned int max_cpus)
 	       ((bogosum + 2500) / (5000/HZ)) % 100);
 }
 
-\f
 void
 smp_percpu_timer_interrupt(struct pt_regs *regs)
 {
@@ -681,7 +680,7 @@ ipi_flush_tlb_mm(void *x)
 void
 flush_tlb_mm(struct mm_struct *mm)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if (mm == current->active_mm) {
 		flush_tlb_current(mm);
@@ -693,7 +692,7 @@ flush_tlb_mm(struct mm_struct *mm)
 				if (mm->context[cpu])
 					mm->context[cpu] = 0;
 			}
-			preempt_enable();
+			put_online_cpus_atomic();
 			return;
 		}
 	}
@@ -702,7 +701,7 @@ flush_tlb_mm(struct mm_struct *mm)
 		printk(KERN_CRIT "flush_tlb_mm: timed out\n");
 	}
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL(flush_tlb_mm);
 
@@ -730,7 +729,7 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 	struct flush_tlb_page_struct data;
 	struct mm_struct *mm = vma->vm_mm;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if (mm == current->active_mm) {
 		flush_tlb_current_page(mm, vma, addr);
@@ -742,7 +741,7 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 				if (mm->context[cpu])
 					mm->context[cpu] = 0;
 			}
-			preempt_enable();
+			put_online_cpus_atomic();
 			return;
 		}
 	}
@@ -755,7 +754,7 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 		printk(KERN_CRIT "flush_tlb_page: timed out\n");
 	}
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 EXPORT_SYMBOL(flush_tlb_page);
 
@@ -786,7 +785,7 @@ flush_icache_user_range(struct vm_area_struct *vma, struct page *page,
 	if ((vma->vm_flags & VM_EXEC) == 0)
 		return;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if (mm == current->active_mm) {
 		__load_new_mm_context(mm);
@@ -798,7 +797,7 @@ flush_icache_user_range(struct vm_area_struct *vma, struct page *page,
 				if (mm->context[cpu])
 					mm->context[cpu] = 0;
 			}
-			preempt_enable();
+			put_online_cpus_atomic();
 			return;
 		}
 	}
@@ -807,5 +806,5 @@ flush_icache_user_range(struct vm_area_struct *vma, struct page *page,
 		printk(KERN_CRIT "flush_icache_page: timed out\n");
 	}
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }

[PATCH v2 32/45] blackfin/smp: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Bob Liu <lliubbo@gmail.com>
Cc: Steven Miao <realmz6@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: uclinux-dist-devel@blackfin.uclinux.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/blackfin/mach-common/smp.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/blackfin/mach-common/smp.c b/arch/blackfin/mach-common/smp.c
index 1bc2ce6..11496cd 100644
--- a/arch/blackfin/mach-common/smp.c
+++ b/arch/blackfin/mach-common/smp.c
@@ -238,13 +238,13 @@ void smp_send_stop(void)
 {
 	cpumask_t callmap;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpumask_copy(&callmap, cpu_online_mask);
 	cpumask_clear_cpu(smp_processor_id(), &callmap);
 	if (!cpumask_empty(&callmap))
 		send_ipi(&callmap, BFIN_IPI_CPU_STOP);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 
 	return;
 }

[PATCH v2 33/45] cris/smp: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-cris-kernel@axis.com
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/cris/arch-v32/kernel/smp.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/cris/arch-v32/kernel/smp.c b/arch/cris/arch-v32/kernel/smp.c
index cdd1202..b2d4612 100644
--- a/arch/cris/arch-v32/kernel/smp.c
+++ b/arch/cris/arch-v32/kernel/smp.c
@@ -13,6 +13,7 @@
 #include <linux/init.h>
 #include <linux/timex.h>
 #include <linux/sched.h>
+#include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/cpumask.h>
 #include <linux/interrupt.h>
@@ -222,6 +223,7 @@ void flush_tlb_common(struct mm_struct* mm, struct vm_area_struct* vma, unsigned
 	unsigned long flags;
 	cpumask_t cpu_mask;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&tlbstate_lock, flags);
 	cpu_mask = (mm == FLUSH_ALL ? cpu_all_mask : *mm_cpumask(mm));
 	cpumask_clear_cpu(smp_processor_id(), &cpu_mask);
@@ -230,6 +232,7 @@ void flush_tlb_common(struct mm_struct* mm, struct vm_area_struct* vma, unsigned
 	flush_addr = addr;
 	send_ipi(IPI_FLUSH_TLB, 1, cpu_mask);
 	spin_unlock_irqrestore(&tlbstate_lock, flags);
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_all(void)
@@ -319,10 +322,12 @@ int smp_call_function(void (*func)(void *info), void *info, int wait)
 	data.info = info;
 	data.wait = wait;
 
+	get_online_cpus_atomic();
 	spin_lock(&call_lock);
 	call_data = &data;
 	ret = send_ipi(IPI_CALL, wait, cpu_mask);
 	spin_unlock(&call_lock);
+	put_online_cpus_atomic();
 
 	return ret;
 }

[PATCH v2 34/45] hexagon/smp: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-hexagon@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/hexagon/kernel/smp.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/hexagon/kernel/smp.c b/arch/hexagon/kernel/smp.c
index 0e364ca..30d4318 100644
--- a/arch/hexagon/kernel/smp.c
+++ b/arch/hexagon/kernel/smp.c
@@ -241,9 +241,12 @@ void smp_send_reschedule(int cpu)
 void smp_send_stop(void)
 {
 	struct cpumask targets;
+
+	get_online_cpus_atomic();
 	cpumask_copy(&targets, cpu_online_mask);
 	cpumask_clear_cpu(smp_processor_id(), &targets);
 	send_ipi(&targets, IPI_CPU_STOP);
+	put_online_cpus_atomic();
 }
 
 void arch_send_call_function_single_ipi(int cpu)

[PATCH v2 35/45] ia64: irq, perfmon: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/ia64/kernel/irq_ia64.c |   15 +++++++++++++++
 arch/ia64/kernel/perfmon.c  |    8 +++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/ia64/kernel/irq_ia64.c b/arch/ia64/kernel/irq_ia64.c
index 1034884..f58b162 100644
--- a/arch/ia64/kernel/irq_ia64.c
+++ b/arch/ia64/kernel/irq_ia64.c
@@ -25,6 +25,7 @@
 #include <linux/ptrace.h>
 #include <linux/signal.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/threads.h>
 #include <linux/bitops.h>
 #include <linux/irq.h>
@@ -160,9 +161,11 @@ int bind_irq_vector(int irq, int vector, cpumask_t domain)
 	unsigned long flags;
 	int ret;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	ret = __bind_irq_vector(irq, vector, domain);
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	return ret;
 }
 
@@ -190,9 +193,11 @@ static void clear_irq_vector(int irq)
 {
 	unsigned long flags;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	__clear_irq_vector(irq);
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 }
 
 int
@@ -204,6 +209,7 @@ ia64_native_assign_irq_vector (int irq)
 
 	vector = -ENOSPC;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	for_each_online_cpu(cpu) {
 		domain = vector_allocation_domain(cpu);
@@ -218,6 +224,7 @@ ia64_native_assign_irq_vector (int irq)
 	BUG_ON(__bind_irq_vector(irq, vector, domain));
  out:
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	return vector;
 }
 
@@ -302,9 +309,11 @@ int irq_prepare_move(int irq, int cpu)
 	unsigned long flags;
 	int ret;
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	ret = __irq_prepare_move(irq, cpu);
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	return ret;
 }
 
@@ -320,11 +329,13 @@ void irq_complete_move(unsigned irq)
 	if (unlikely(cpu_isset(smp_processor_id(), cfg->old_domain)))
 		return;
 
+	get_online_cpus_atomic();
 	cpumask_and(&cleanup_mask, &cfg->old_domain, cpu_online_mask);
 	cfg->move_cleanup_count = cpus_weight(cleanup_mask);
 	for_each_cpu_mask(i, cleanup_mask)
 		platform_send_ipi(i, IA64_IRQ_MOVE_VECTOR, IA64_IPI_DM_INT, 0);
 	cfg->move_in_progress = 0;
+	put_online_cpus_atomic();
 }
 
 static irqreturn_t smp_irq_move_cleanup_interrupt(int irq, void *dev_id)
@@ -393,10 +404,12 @@ void destroy_and_reserve_irq(unsigned int irq)
 
 	dynamic_irq_cleanup(irq);
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	__clear_irq_vector(irq);
 	irq_status[irq] = IRQ_RSVD;
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 }
 
 /*
@@ -409,6 +422,7 @@ int create_irq(void)
 	cpumask_t domain = CPU_MASK_NONE;
 
 	irq = vector = -ENOSPC;
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&vector_lock, flags);
 	for_each_online_cpu(cpu) {
 		domain = vector_allocation_domain(cpu);
@@ -424,6 +438,7 @@ int create_irq(void)
 	BUG_ON(__bind_irq_vector(irq, vector, domain));
  out:
 	spin_unlock_irqrestore(&vector_lock, flags);
+	put_online_cpus_atomic();
 	if (irq >= 0)
 		dynamic_irq_init(irq);
 	return irq;
diff --git a/arch/ia64/kernel/perfmon.c b/arch/ia64/kernel/perfmon.c
index 9ea25fc..16c8303 100644
--- a/arch/ia64/kernel/perfmon.c
+++ b/arch/ia64/kernel/perfmon.c
@@ -6476,9 +6476,12 @@ pfm_install_alt_pmu_interrupt(pfm_intr_handler_desc_t *hdl)
 	/* do the easy test first */
 	if (pfm_alt_intr_handler) return -EBUSY;
 
+	get_online_cpus_atomic();
+
 	/* one at a time in the install or remove, just fail the others */
 	if (!spin_trylock(&pfm_alt_install_check)) {
-		return -EBUSY;
+		ret = -EBUSY;
+		goto out;
 	}
 
 	/* reserve our session */
@@ -6498,6 +6501,7 @@ pfm_install_alt_pmu_interrupt(pfm_intr_handler_desc_t *hdl)
 	pfm_alt_intr_handler = hdl;
 
 	spin_unlock(&pfm_alt_install_check);
+	put_online_cpus_atomic();
 
 	return 0;
 
@@ -6510,6 +6514,8 @@ cleanup_reserve:
 	}
 
 	spin_unlock(&pfm_alt_install_check);
+out:
+	put_online_cpus_atomic();
 
 	return ret;
 }

[PATCH v2 36/45] ia64: smp, tlb: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/ia64/kernel/smp.c |   12 ++++++------
 arch/ia64/mm/tlb.c     |    4 ++--
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/ia64/kernel/smp.c b/arch/ia64/kernel/smp.c
index 9fcd4e6..25991ba 100644
--- a/arch/ia64/kernel/smp.c
+++ b/arch/ia64/kernel/smp.c
@@ -24,6 +24,7 @@
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/kernel_stat.h>
 #include <linux/mm.h>
 #include <linux/cache.h>
@@ -259,8 +260,7 @@ smp_flush_tlb_cpumask(cpumask_t xcpumask)
 	cpumask_t cpumask = xcpumask;
 	int mycpu, cpu, flush_mycpu = 0;
 
-	preempt_disable();
-	mycpu = smp_processor_id();
+	mycpu = get_online_cpus_atomic();
 
 	for_each_cpu_mask(cpu, cpumask)
 		counts[cpu] = local_tlb_flush_counts[cpu].count & 0xffff;
@@ -280,7 +280,7 @@ smp_flush_tlb_cpumask(cpumask_t xcpumask)
 		while(counts[cpu] == (local_tlb_flush_counts[cpu].count & 0xffff))
 			udelay(FLUSH_DELAY);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 void
@@ -293,12 +293,12 @@ void
 smp_flush_tlb_mm (struct mm_struct *mm)
 {
 	cpumask_var_t cpus;
-	preempt_disable();
+	get_online_cpus_atomic();
 	/* this happens for the common case of a single-threaded fork():  */
 	if (likely(mm == current->active_mm && atomic_read(&mm->mm_users) == 1))
 	{
 		local_finish_flush_tlb_mm(mm);
-		preempt_enable();
+		put_online_cpus_atomic();
 		return;
 	}
 	if (!alloc_cpumask_var(&cpus, GFP_ATOMIC)) {
@@ -313,7 +313,7 @@ smp_flush_tlb_mm (struct mm_struct *mm)
 	local_irq_disable();
 	local_finish_flush_tlb_mm(mm);
 	local_irq_enable();
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 void arch_send_call_function_single_ipi(int cpu)
diff --git a/arch/ia64/mm/tlb.c b/arch/ia64/mm/tlb.c
index ed61297..8c55ef5 100644
--- a/arch/ia64/mm/tlb.c
+++ b/arch/ia64/mm/tlb.c
@@ -87,11 +87,11 @@ wrap_mmu_context (struct mm_struct *mm)
 	 * can't call flush_tlb_all() here because of race condition
 	 * with O(1) scheduler [EF]
 	 */
-	cpu = get_cpu(); /* prevent preemption/migration */
+	cpu = get_online_cpus_atomic(); /* prevent preemption/migration */
 	for_each_online_cpu(i)
 		if (i != cpu)
 			per_cpu(ia64_need_tlb_flush, i) = 1;
-	put_cpu();
+	put_online_cpus_atomic();
 	local_flush_tlb_all();
 }
 

[PATCH v2 37/45] m32r: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: linux-m32r@ml.linux-m32r.org
Cc: linux-m32r-ja@ml.linux-m32r.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/m32r/kernel/smp.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/m32r/kernel/smp.c b/arch/m32r/kernel/smp.c
index ce7aea3..ffafdba 100644
--- a/arch/m32r/kernel/smp.c
+++ b/arch/m32r/kernel/smp.c
@@ -151,7 +151,7 @@ void smp_flush_cache_all(void)
 	cpumask_t cpumask;
 	unsigned long *mask;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpumask_copy(&cpumask, cpu_online_mask);
 	cpumask_clear_cpu(smp_processor_id(), &cpumask);
 	spin_lock(&flushcache_lock);
@@ -162,7 +162,7 @@ void smp_flush_cache_all(void)
 	while (flushcache_cpumask)
 		mb();
 	spin_unlock(&flushcache_lock);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 void smp_flush_cache_all_interrupt(void)
@@ -197,12 +197,12 @@ void smp_flush_tlb_all(void)
 {
 	unsigned long flags;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	local_irq_save(flags);
 	__flush_tlb_all();
 	local_irq_restore(flags);
 	smp_call_function(flush_tlb_all_ipi, NULL, 1);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /*==========================================================================*
@@ -250,7 +250,7 @@ void smp_flush_tlb_mm(struct mm_struct *mm)
 	unsigned long *mmc;
 	unsigned long flags;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpu_id = smp_processor_id();
 	mmc = &mm->context[cpu_id];
 	cpumask_copy(&cpu_mask, mm_cpumask(mm));
@@ -268,7 +268,7 @@ void smp_flush_tlb_mm(struct mm_struct *mm)
 	if (!cpumask_empty(&cpu_mask))
 		flush_tlb_others(cpu_mask, mm, NULL, FLUSH_ALL);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /*==========================================================================*
@@ -320,7 +320,7 @@ void smp_flush_tlb_page(struct vm_area_struct *vma, unsigned long va)
 	unsigned long *mmc;
 	unsigned long flags;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpu_id = smp_processor_id();
 	mmc = &mm->context[cpu_id];
 	cpumask_copy(&cpu_mask, mm_cpumask(mm));
@@ -341,7 +341,7 @@ void smp_flush_tlb_page(struct vm_area_struct *vma, unsigned long va)
 	if (!cpumask_empty(&cpu_mask))
 		flush_tlb_others(cpu_mask, mm, vma, va);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /*==========================================================================*

[PATCH v2 38/45] MIPS: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: David Daney <david.daney@cavium.com>
Cc: Yong Zhang <yong.zhang0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sanjay Lal <sanjayl@kymasys.com>
Cc: "Steven J. Hill" <sjhill@mips.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/mips/kernel/cevt-smtc.c |    7 +++++++
 arch/mips/kernel/smp.c       |   16 ++++++++--------
 arch/mips/kernel/smtc.c      |   12 ++++++++++++
 arch/mips/mm/c-octeon.c      |    4 ++--
 4 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/arch/mips/kernel/cevt-smtc.c b/arch/mips/kernel/cevt-smtc.c
index 9de5ed7..2e6c0cd 100644
--- a/arch/mips/kernel/cevt-smtc.c
+++ b/arch/mips/kernel/cevt-smtc.c
@@ -11,6 +11,7 @@
 #include <linux/interrupt.h>
 #include <linux/percpu.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/irq.h>
 
 #include <asm/smtc_ipi.h>
@@ -84,6 +85,8 @@ static int mips_next_event(unsigned long delta,
 	unsigned long nextcomp = 0L;
 	int vpe = current_cpu_data.vpe_id;
 	int cpu = smp_processor_id();
+
+	get_online_cpus_atomic();
 	local_irq_save(flags);
 	mtflags = dmt();
 
@@ -164,6 +167,7 @@ static int mips_next_event(unsigned long delta,
 	}
 	emt(mtflags);
 	local_irq_restore(flags);
+	put_online_cpus_atomic();
 	return 0;
 }
 
@@ -177,6 +181,7 @@ void smtc_distribute_timer(int vpe)
 	unsigned long nextstamp;
 	unsigned long reference;
 
+	get_online_cpus_atomic();
 
 repeat:
 	nextstamp = 0L;
@@ -229,6 +234,8 @@ repeat:
 			> (unsigned long)LONG_MAX)
 				goto repeat;
 	}
+
+	put_online_cpus_atomic();
 }
 
 
diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
index 6e7862a..be152b6 100644
--- a/arch/mips/kernel/smp.c
+++ b/arch/mips/kernel/smp.c
@@ -250,12 +250,12 @@ static inline void smp_on_other_tlbs(void (*func) (void *info), void *info)
 
 static inline void smp_on_each_tlb(void (*func) (void *info), void *info)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	smp_on_other_tlbs(func, info);
 	func(info);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /*
@@ -273,7 +273,7 @@ static inline void smp_on_each_tlb(void (*func) (void *info), void *info)
 
 void flush_tlb_mm(struct mm_struct *mm)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if ((atomic_read(&mm->mm_users) != 1) || (current->mm != mm)) {
 		smp_on_other_tlbs(flush_tlb_mm_ipi, mm);
@@ -287,7 +287,7 @@ void flush_tlb_mm(struct mm_struct *mm)
 	}
 	local_flush_tlb_mm(mm);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 struct flush_tlb_data {
@@ -307,7 +307,7 @@ void flush_tlb_range(struct vm_area_struct *vma, unsigned long start, unsigned l
 {
 	struct mm_struct *mm = vma->vm_mm;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	if ((atomic_read(&mm->mm_users) != 1) || (current->mm != mm)) {
 		struct flush_tlb_data fd = {
 			.vma = vma,
@@ -325,7 +325,7 @@ void flush_tlb_range(struct vm_area_struct *vma, unsigned long start, unsigned l
 		}
 	}
 	local_flush_tlb_range(vma, start, end);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 static void flush_tlb_kernel_range_ipi(void *info)
@@ -354,7 +354,7 @@ static void flush_tlb_page_ipi(void *info)
 
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 	if ((atomic_read(&vma->vm_mm->mm_users) != 1) || (current->mm != vma->vm_mm)) {
 		struct flush_tlb_data fd = {
 			.vma = vma,
@@ -371,7 +371,7 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 		}
 	}
 	local_flush_tlb_page(vma, page);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 static void flush_tlb_one_ipi(void *info)
diff --git a/arch/mips/kernel/smtc.c b/arch/mips/kernel/smtc.c
index 75a4fd7..3cda8eb 100644
--- a/arch/mips/kernel/smtc.c
+++ b/arch/mips/kernel/smtc.c
@@ -21,6 +21,7 @@
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/cpumask.h>
 #include <linux/interrupt.h>
 #include <linux/kernel_stat.h>
@@ -1143,6 +1144,8 @@ static irqreturn_t ipi_interrupt(int irq, void *dev_idm)
 	 * for the current TC, so we ought not to have to do it explicitly here.
 	 */
 
+	get_online_cpus_atomic();
+
 	for_each_online_cpu(cpu) {
 		if (cpu_data[cpu].vpe_id != my_vpe)
 			continue;
@@ -1180,6 +1183,8 @@ static irqreturn_t ipi_interrupt(int irq, void *dev_idm)
 		}
 	}
 
+	put_online_cpus_atomic();
+
 	return IRQ_HANDLED;
 }
 
@@ -1383,6 +1388,7 @@ void smtc_get_new_mmu_context(struct mm_struct *mm, unsigned long cpu)
 	 * them, let's be really careful...
 	 */
 
+	get_online_cpus_atomic();
 	local_irq_save(flags);
 	if (smtc_status & SMTC_TLB_SHARED) {
 		mtflags = dvpe();
@@ -1438,6 +1444,7 @@ void smtc_get_new_mmu_context(struct mm_struct *mm, unsigned long cpu)
 	else
 		emt(mtflags);
 	local_irq_restore(flags);
+	put_online_cpus_atomic();
 }
 
 /*
@@ -1496,6 +1503,7 @@ void smtc_cflush_lockdown(void)
 {
 	int cpu;
 
+	get_online_cpus_atomic();
 	for_each_online_cpu(cpu) {
 		if (cpu != smp_processor_id()) {
 			settc(cpu_data[cpu].tc_id);
@@ -1504,6 +1512,7 @@ void smtc_cflush_lockdown(void)
 		}
 	}
 	mips_ihb();
+	put_online_cpus_atomic();
 }
 
 /* It would be cheating to change the cpu_online states during a flush! */
@@ -1512,6 +1521,8 @@ void smtc_cflush_release(void)
 {
 	int cpu;
 
+	get_online_cpus_atomic();
+
 	/*
 	 * Start with a hazard barrier to ensure
 	 * that all CACHE ops have played through.
@@ -1525,4 +1536,5 @@ void smtc_cflush_release(void)
 		}
 	}
 	mips_ihb();
+	put_online_cpus_atomic();
 }
diff --git a/arch/mips/mm/c-octeon.c b/arch/mips/mm/c-octeon.c
index 8557fb5..8e1bcf6 100644
--- a/arch/mips/mm/c-octeon.c
+++ b/arch/mips/mm/c-octeon.c
@@ -73,7 +73,7 @@ static void octeon_flush_icache_all_cores(struct vm_area_struct *vma)
 	mb();
 	octeon_local_flush_icache();
 #ifdef CONFIG_SMP
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpu = smp_processor_id();
 
 	/*
@@ -88,7 +88,7 @@ static void octeon_flush_icache_all_cores(struct vm_area_struct *vma)
 	for_each_cpu(cpu, &mask)
 		octeon_send_ipi_single(cpu, SMP_ICACHE_FLUSH);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 #endif
 }
 

Re: [PATCH v2 38/45] MIPS: Use get/put_online_cpus_atomic() to prevent CPU offline

[Ralf Baechle]

On Wed, Jun 26, 2013 at 02:02:57AM +0530, Srivatsa S. Bhat wrote:

> Once stop_machine() is gone from the CPU offline path, we won't be able
> to depend on disabling preemption to prevent CPUs from going offline
> from under us.
> 
> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
> offline, while invoking from atomic context.

I think the same change also needs to be applied to r4k_on_each_cpu() in
arch/mips/mm/c-r4k.c which currently looks like:

static inline void r4k_on_each_cpu(void (*func) (void *info), void *info)
{
        preempt_disable();

#if !defined(CONFIG_MIPS_MT_SMP) && !defined(CONFIG_MIPS_MT_SMTC)
        smp_call_function(func, info, 1);
#endif
        func(info);
        preempt_enable();
}

This is a slightly specialized version of on_each_cpu() which only calls
out to other CPUs in actual multi-core environments and also - unlike
on_each_cpu() doesn't disable interrupts for the sake of better
interrupt latencies.

Which reminds me ...

Andrew, I was wondering why did 78eef01b0fae087c5fadbd85dd4fe2918c3a015f
[[PATCH] on_each_cpu(): disable local interrupts] disable interrupts?
The log is:

----- snip -----
    When on_each_cpu() runs the callback on other CPUs, it runs with local
    interrupts disabled.  So we should run the function with local interrupts
    disabled on this CPU, too.
    
    And do the same for UP, so the callback is run in the same environment on bo
    UP and SMP.  (strictly it should do preempt_disable() too, but I think
    local_irq_disable is sufficiently equivalent).
[...]
----- snip -----

I'm not entirely convinced the symmetry between UP and SMP environments is
really worth it.  Would anybody mind removing the local_irq_disable() ...
local_irq_enable() from on_each_cpu()?

  Ralf

Re: [PATCH v2 38/45] MIPS: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

On 06/26/2013 07:09 PM, Ralf Baechle wrote:
> On Wed, Jun 26, 2013 at 02:02:57AM +0530, Srivatsa S. Bhat wrote:
> 
>> Once stop_machine() is gone from the CPU offline path, we won't be able
>> to depend on disabling preemption to prevent CPUs from going offline
>> from under us.
>>
>> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
>> offline, while invoking from atomic context.
> 
> I think the same change also needs to be applied to r4k_on_each_cpu() in
> arch/mips/mm/c-r4k.c which currently looks like:
> 
> static inline void r4k_on_each_cpu(void (*func) (void *info), void *info)
> {
>         preempt_disable();
> 
> #if !defined(CONFIG_MIPS_MT_SMP) && !defined(CONFIG_MIPS_MT_SMTC)
>         smp_call_function(func, info, 1);
> #endif
>         func(info);
>         preempt_enable();
> }
> 

Thanks for pointing this out! I'll include changes to this code in my
next version.

Regards,
Srivatsa S. Bhat

> This is a slightly specialized version of on_each_cpu() which only calls
> out to other CPUs in actual multi-core environments and also - unlike
> on_each_cpu() doesn't disable interrupts for the sake of better
> interrupt latencies.
> 
> Which reminds me ...
> 
> Andrew, I was wondering why did 78eef01b0fae087c5fadbd85dd4fe2918c3a015f
> [[PATCH] on_each_cpu(): disable local interrupts] disable interrupts?
> The log is:
> 
> ----- snip -----
>     When on_each_cpu() runs the callback on other CPUs, it runs with local
>     interrupts disabled.  So we should run the function with local interrupts
>     disabled on this CPU, too.
>     
>     And do the same for UP, so the callback is run in the same environment on bo
>     UP and SMP.  (strictly it should do preempt_disable() too, but I think
>     local_irq_disable is sufficiently equivalent).
> [...]
> ----- snip -----
> 
> I'm not entirely convinced the symmetry between UP and SMP environments is
> really worth it.  Would anybody mind removing the local_irq_disable() ...
> local_irq_enable() from on_each_cpu()?
> 

[PATCH v2 39/45] mn10300: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: David Howells <dhowells@redhat.com>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: linux-am33-list@redhat.com
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/mn10300/mm/cache-smp.c |    3 +++
 arch/mn10300/mm/tlb-smp.c   |   17 +++++++++--------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/mn10300/mm/cache-smp.c b/arch/mn10300/mm/cache-smp.c
index 2d23b9e..406357d 100644
--- a/arch/mn10300/mm/cache-smp.c
+++ b/arch/mn10300/mm/cache-smp.c
@@ -13,6 +13,7 @@
 #include <linux/mman.h>
 #include <linux/threads.h>
 #include <linux/interrupt.h>
+#include <linux/cpu.h>
 #include <asm/page.h>
 #include <asm/pgtable.h>
 #include <asm/processor.h>
@@ -91,6 +92,7 @@ void smp_cache_interrupt(void)
 void smp_cache_call(unsigned long opr_mask,
 		    unsigned long start, unsigned long end)
 {
+	get_online_cpus_atomic();
 	smp_cache_mask = opr_mask;
 	smp_cache_start = start;
 	smp_cache_end = end;
@@ -102,4 +104,5 @@ void smp_cache_call(unsigned long opr_mask,
 	while (!cpumask_empty(&smp_cache_ipi_map))
 		/* nothing. lockup detection does not belong here */
 		mb();
+	put_online_cpus_atomic();
 }
diff --git a/arch/mn10300/mm/tlb-smp.c b/arch/mn10300/mm/tlb-smp.c
index 3e57faf..8856fd3 100644
--- a/arch/mn10300/mm/tlb-smp.c
+++ b/arch/mn10300/mm/tlb-smp.c
@@ -23,6 +23,7 @@
 #include <linux/sched.h>
 #include <linux/profile.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <asm/tlbflush.h>
 #include <asm/bitops.h>
 #include <asm/processor.h>
@@ -61,7 +62,7 @@ void smp_flush_tlb(void *unused)
 {
 	unsigned long cpu_id;
 
-	cpu_id = get_cpu();
+	cpu_id = get_online_cpus_atomic();
 
 	if (!cpumask_test_cpu(cpu_id, &flush_cpumask))
 		/* This was a BUG() but until someone can quote me the line
@@ -82,7 +83,7 @@ void smp_flush_tlb(void *unused)
 	cpumask_clear_cpu(cpu_id, &flush_cpumask);
 	smp_mb__after_clear_bit();
 out:
-	put_cpu();
+	put_online_cpus_atomic();
 }
 
 /**
@@ -144,7 +145,7 @@ void flush_tlb_mm(struct mm_struct *mm)
 {
 	cpumask_t cpu_mask;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpumask_copy(&cpu_mask, mm_cpumask(mm));
 	cpumask_clear_cpu(smp_processor_id(), &cpu_mask);
 
@@ -152,7 +153,7 @@ void flush_tlb_mm(struct mm_struct *mm)
 	if (!cpumask_empty(&cpu_mask))
 		flush_tlb_others(cpu_mask, mm, FLUSH_ALL);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /**
@@ -163,7 +164,7 @@ void flush_tlb_current_task(void)
 	struct mm_struct *mm = current->mm;
 	cpumask_t cpu_mask;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpumask_copy(&cpu_mask, mm_cpumask(mm));
 	cpumask_clear_cpu(smp_processor_id(), &cpu_mask);
 
@@ -171,7 +172,7 @@ void flush_tlb_current_task(void)
 	if (!cpumask_empty(&cpu_mask))
 		flush_tlb_others(cpu_mask, mm, FLUSH_ALL);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /**
@@ -184,7 +185,7 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long va)
 	struct mm_struct *mm = vma->vm_mm;
 	cpumask_t cpu_mask;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	cpumask_copy(&cpu_mask, mm_cpumask(mm));
 	cpumask_clear_cpu(smp_processor_id(), &cpu_mask);
 
@@ -192,7 +193,7 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long va)
 	if (!cpumask_empty(&cpu_mask))
 		flush_tlb_others(cpu_mask, mm, va);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 /**

[PATCH v2 40/45] powerpc, irq: Use GFP_ATOMIC allocations in atomic context

[Srivatsa S. Bhat]

The function migrate_irqs() is called with interrupts disabled
and hence its not safe to do GFP_KERNEL allocations inside it,
because they can sleep. So change the gfp mask to GFP_ATOMIC.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ian Munsie <imunsie@au1.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/powerpc/kernel/irq.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index ea185e0..ca39bac 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -412,7 +412,7 @@ void migrate_irqs(void)
 	cpumask_var_t mask;
 	const struct cpumask *map = cpu_online_mask;
 
-	alloc_cpumask_var(&mask, GFP_KERNEL);
+	alloc_cpumask_var(&mask, GFP_ATOMIC);
 
 	for_each_irq_desc(irq, desc) {
 		struct irq_data *data;

[PATCH v2 41/45] powerpc: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Gleb Natapov <gleb@redhat.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Zhao Chenhui <chenhui.zhao@freescale.com>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: kvm@vger.kernel.org
Cc: kvm-ppc@vger.kernel.org
Cc: oprofile-list@lists.sf.net
Cc: cbe-oss-dev@lists.ozlabs.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/powerpc/kernel/irq.c                  |    7 ++++++-
 arch/powerpc/kernel/machine_kexec_64.c     |    4 ++--
 arch/powerpc/kernel/smp.c                  |    2 ++
 arch/powerpc/kvm/book3s_hv.c               |    5 +++--
 arch/powerpc/mm/mmu_context_nohash.c       |    3 +++
 arch/powerpc/oprofile/cell/spu_profiler.c  |    3 +++
 arch/powerpc/oprofile/cell/spu_task_sync.c |    4 ++++
 arch/powerpc/oprofile/op_model_cell.c      |    6 ++++++
 8 files changed, 29 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index ca39bac..41e9961 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -45,6 +45,7 @@
 #include <linux/irq.h>
 #include <linux/seq_file.h>
 #include <linux/cpumask.h>
+#include <linux/cpu.h>
 #include <linux/profile.h>
 #include <linux/bitops.h>
 #include <linux/list.h>
@@ -410,7 +411,10 @@ void migrate_irqs(void)
 	unsigned int irq;
 	static int warned;
 	cpumask_var_t mask;
-	const struct cpumask *map = cpu_online_mask;
+	const struct cpumask *map;
+
+	get_online_cpus_atomic();
+	map = cpu_online_mask;
 
 	alloc_cpumask_var(&mask, GFP_ATOMIC);
 
@@ -436,6 +440,7 @@ void migrate_irqs(void)
 	}
 
 	free_cpumask_var(mask);
+	put_online_cpus_atomic();
 
 	local_irq_enable();
 	mdelay(1);
diff --git a/arch/powerpc/kernel/machine_kexec_64.c b/arch/powerpc/kernel/machine_kexec_64.c
index 611acdf..38f6d75 100644
--- a/arch/powerpc/kernel/machine_kexec_64.c
+++ b/arch/powerpc/kernel/machine_kexec_64.c
@@ -187,7 +187,7 @@ static void kexec_prepare_cpus_wait(int wait_state)
 	int my_cpu, i, notified=-1;
 
 	hw_breakpoint_disable();
-	my_cpu = get_cpu();
+	my_cpu = get_online_cpus_atomic();
 	/* Make sure each CPU has at least made it to the state we need.
 	 *
 	 * FIXME: There is a (slim) chance of a problem if not all of the CPUs
@@ -266,7 +266,7 @@ static void kexec_prepare_cpus(void)
 	 */
 	kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE);
 
-	put_cpu();
+	put_online_cpus_atomic();
 }
 
 #else /* ! SMP */
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index ee7ac5e..2123bec 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -277,9 +277,11 @@ void smp_send_debugger_break(void)
 	if (unlikely(!smp_ops))
 		return;
 
+	get_online_cpus_atomic();
 	for_each_online_cpu(cpu)
 		if (cpu != me)
 			do_message_pass(cpu, PPC_MSG_DEBUGGER_BREAK);
+	put_online_cpus_atomic();
 }
 #endif
 
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 2efa9dd..9d8a973 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -28,6 +28,7 @@
 #include <linux/fs.h>
 #include <linux/anon_inodes.h>
 #include <linux/cpumask.h>
+#include <linux/cpu.h>
 #include <linux/spinlock.h>
 #include <linux/page-flags.h>
 #include <linux/srcu.h>
@@ -78,7 +79,7 @@ void kvmppc_fast_vcpu_kick(struct kvm_vcpu *vcpu)
 		++vcpu->stat.halt_wakeup;
 	}
 
-	me = get_cpu();
+	me = get_online_cpus_atomic();
 
 	/* CPU points to the first thread of the core */
 	if (cpu != me && cpu >= 0 && cpu < nr_cpu_ids) {
@@ -88,7 +89,7 @@ void kvmppc_fast_vcpu_kick(struct kvm_vcpu *vcpu)
 		else if (cpu_online(cpu))
 			smp_send_reschedule(cpu);
 	}
-	put_cpu();
+	put_online_cpus_atomic();
 }
 
 /*
diff --git a/arch/powerpc/mm/mmu_context_nohash.c b/arch/powerpc/mm/mmu_context_nohash.c
index e779642..c7bdcb4 100644
--- a/arch/powerpc/mm/mmu_context_nohash.c
+++ b/arch/powerpc/mm/mmu_context_nohash.c
@@ -194,6 +194,8 @@ void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
 	unsigned int i, id, cpu = smp_processor_id();
 	unsigned long *map;
 
+	get_online_cpus_atomic();
+
 	/* No lockless fast path .. yet */
 	raw_spin_lock(&context_lock);
 
@@ -280,6 +282,7 @@ void switch_mmu_context(struct mm_struct *prev, struct mm_struct *next)
 	pr_hardcont(" -> %d\n", id);
 	set_context(id, next->pgd);
 	raw_spin_unlock(&context_lock);
+	put_online_cpus_atomic();
 }
 
 /*
diff --git a/arch/powerpc/oprofile/cell/spu_profiler.c b/arch/powerpc/oprofile/cell/spu_profiler.c
index b129d00..ab6e6c1 100644
--- a/arch/powerpc/oprofile/cell/spu_profiler.c
+++ b/arch/powerpc/oprofile/cell/spu_profiler.c
@@ -14,6 +14,7 @@
 
 #include <linux/hrtimer.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/slab.h>
 #include <asm/cell-pmu.h>
 #include <asm/time.h>
@@ -142,6 +143,7 @@ static enum hrtimer_restart profile_spus(struct hrtimer *timer)
 	if (!spu_prof_running)
 		goto stop;
 
+	get_online_cpus_atomic();
 	for_each_online_cpu(cpu) {
 		if (cbe_get_hw_thread_id(cpu))
 			continue;
@@ -177,6 +179,7 @@ static enum hrtimer_restart profile_spus(struct hrtimer *timer)
 				       oprof_spu_smpl_arry_lck_flags);
 
 	}
+	put_online_cpus_atomic();
 	smp_wmb();	/* insure spu event buffer updates are written */
 			/* don't want events intermingled... */
 
diff --git a/arch/powerpc/oprofile/cell/spu_task_sync.c b/arch/powerpc/oprofile/cell/spu_task_sync.c
index 28f1af2..8464ef6 100644
--- a/arch/powerpc/oprofile/cell/spu_task_sync.c
+++ b/arch/powerpc/oprofile/cell/spu_task_sync.c
@@ -28,6 +28,7 @@
 #include <linux/oprofile.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
+#include <linux/cpu.h>
 #include "pr_util.h"
 
 #define RELEASE_ALL 9999
@@ -448,11 +449,14 @@ static int number_of_online_nodes(void)
 {
         u32 cpu; u32 tmp;
         int nodes = 0;
+
+	get_online_cpus_atomic();
         for_each_online_cpu(cpu) {
                 tmp = cbe_cpu_to_node(cpu) + 1;
                 if (tmp > nodes)
                         nodes++;
         }
+	put_online_cpus_atomic();
         return nodes;
 }
 
diff --git a/arch/powerpc/oprofile/op_model_cell.c b/arch/powerpc/oprofile/op_model_cell.c
index b9589c1..c9bb028 100644
--- a/arch/powerpc/oprofile/op_model_cell.c
+++ b/arch/powerpc/oprofile/op_model_cell.c
@@ -22,6 +22,7 @@
 #include <linux/oprofile.h>
 #include <linux/percpu.h>
 #include <linux/smp.h>
+#include <linux/cpu.h>
 #include <linux/spinlock.h>
 #include <linux/timer.h>
 #include <asm/cell-pmu.h>
@@ -463,6 +464,7 @@ static void cell_virtual_cntr(unsigned long data)
 	 * not both playing with the counters on the same node.
 	 */
 
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&cntr_lock, flags);
 
 	prev_hdw_thread = hdw_thread;
@@ -550,6 +552,7 @@ static void cell_virtual_cntr(unsigned long data)
 	}
 
 	spin_unlock_irqrestore(&cntr_lock, flags);
+	put_online_cpus_atomic();
 
 	mod_timer(&timer_virt_cntr, jiffies + HZ / 10);
 }
@@ -608,6 +611,8 @@ static void spu_evnt_swap(unsigned long data)
 	/* Make sure spu event interrupt handler and spu event swap
 	 * don't access the counters simultaneously.
 	 */
+
+	get_online_cpus_atomic();
 	spin_lock_irqsave(&cntr_lock, flags);
 
 	cur_spu_evnt_phys_spu_indx = spu_evnt_phys_spu_indx;
@@ -673,6 +678,7 @@ static void spu_evnt_swap(unsigned long data)
 	}
 
 	spin_unlock_irqrestore(&cntr_lock, flags);
+	put_online_cpus_atomic();
 
 	/* swap approximately every 0.1 seconds */
 	mod_timer(&timer_spu_event_swap, jiffies + HZ / 25);

[PATCH v2 42/45] powerpc: Use get/put_online_cpus_atomic() to avoid false-positive warning

[Srivatsa S. Bhat]

Bringing a secondary CPU online is a special case in which, accessing
the cpu_online_mask is safe, even though that task (which running on the
CPU coming online) is not the hotplug writer.

It is a little hard to teach this to the debugging checks under
CONFIG_DEBUG_HOTPLUG_CPU. But luckily powerpc is one of the few places
where the CPU coming online traverses the cpu_online_mask before fully
coming online. So wrap that part under get/put_online_cpus_atomic(), to
avoid false-positive warnings from the CPU hotplug debug code.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Zhao Chenhui <chenhui.zhao@freescale.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/powerpc/kernel/smp.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 2123bec..59c9a09 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -657,6 +657,7 @@ __cpuinit void start_secondary(void *unused)
 		cpumask_set_cpu(base + i, cpu_core_mask(cpu));
 	}
 	l2_cache = cpu_to_l2cache(cpu);
+	get_online_cpus_atomic();
 	for_each_online_cpu(i) {
 		struct device_node *np = cpu_to_l2cache(i);
 		if (!np)
@@ -667,6 +668,7 @@ __cpuinit void start_secondary(void *unused)
 		}
 		of_node_put(np);
 	}
+	put_online_cpus_atomic();
 	of_node_put(l2_cache);
 
 	local_irq_enable();

[PATCH v2 43/45] sh: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-sh@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/sh/kernel/smp.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
index 4569645..42ec182 100644
--- a/arch/sh/kernel/smp.c
+++ b/arch/sh/kernel/smp.c
@@ -357,7 +357,7 @@ static void flush_tlb_mm_ipi(void *mm)
  */
 void flush_tlb_mm(struct mm_struct *mm)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 
 	if ((atomic_read(&mm->mm_users) != 1) || (current->mm != mm)) {
 		smp_call_function(flush_tlb_mm_ipi, (void *)mm, 1);
@@ -369,7 +369,7 @@ void flush_tlb_mm(struct mm_struct *mm)
 	}
 	local_flush_tlb_mm(mm);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 struct flush_tlb_data {
@@ -390,7 +390,7 @@ void flush_tlb_range(struct vm_area_struct *vma,
 {
 	struct mm_struct *mm = vma->vm_mm;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 	if ((atomic_read(&mm->mm_users) != 1) || (current->mm != mm)) {
 		struct flush_tlb_data fd;
 
@@ -405,7 +405,7 @@ void flush_tlb_range(struct vm_area_struct *vma,
 				cpu_context(i, mm) = 0;
 	}
 	local_flush_tlb_range(vma, start, end);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 static void flush_tlb_kernel_range_ipi(void *info)
@@ -433,7 +433,7 @@ static void flush_tlb_page_ipi(void *info)
 
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 {
-	preempt_disable();
+	get_online_cpus_atomic();
 	if ((atomic_read(&vma->vm_mm->mm_users) != 1) ||
 	    (current->mm != vma->vm_mm)) {
 		struct flush_tlb_data fd;
@@ -448,7 +448,7 @@ void flush_tlb_page(struct vm_area_struct *vma, unsigned long page)
 				cpu_context(i, vma->vm_mm) = 0;
 	}
 	local_flush_tlb_page(vma, page);
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 static void flush_tlb_one_ipi(void *info)

[PATCH v2 44/45] sparc: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: sparclinux@vger.kernel.org
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/sparc/kernel/smp_64.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/arch/sparc/kernel/smp_64.c b/arch/sparc/kernel/smp_64.c
index 77539ed..4f71a95 100644
--- a/arch/sparc/kernel/smp_64.c
+++ b/arch/sparc/kernel/smp_64.c
@@ -792,7 +792,9 @@ static void smp_cross_call_masked(unsigned long *func, u32 ctx, u64 data1, u64 d
 /* Send cross call to all processors except self. */
 static void smp_cross_call(unsigned long *func, u32 ctx, u64 data1, u64 data2)
 {
+	get_online_cpus_atomic();
 	smp_cross_call_masked(func, ctx, data1, data2, cpu_online_mask);
+	put_online_cpus_atomic();
 }
 
 extern unsigned long xcall_sync_tick;
@@ -896,7 +898,7 @@ void smp_flush_dcache_page_impl(struct page *page, int cpu)
 	atomic_inc(&dcpage_flushes);
 #endif
 
-	this_cpu = get_cpu();
+	this_cpu = get_online_cpus_atomic();
 
 	if (cpu == this_cpu) {
 		__local_flush_dcache_page(page);
@@ -922,7 +924,7 @@ void smp_flush_dcache_page_impl(struct page *page, int cpu)
 		}
 	}
 
-	put_cpu();
+	put_online_cpus_atomic();
 }
 
 void flush_dcache_page_all(struct mm_struct *mm, struct page *page)
@@ -933,7 +935,7 @@ void flush_dcache_page_all(struct mm_struct *mm, struct page *page)
 	if (tlb_type == hypervisor)
 		return;
 
-	preempt_disable();
+	get_online_cpus_atomic();
 
 #ifdef CONFIG_DEBUG_DCFLUSH
 	atomic_inc(&dcpage_flushes);
@@ -958,7 +960,7 @@ void flush_dcache_page_all(struct mm_struct *mm, struct page *page)
 	}
 	__local_flush_dcache_page(page);
 
-	preempt_enable();
+	put_online_cpus_atomic();
 }
 
 void __irq_entry smp_new_mmu_context_version_client(int irq, struct pt_regs *regs)
@@ -1150,6 +1152,7 @@ void smp_capture(void)
 {
 	int result = atomic_add_ret(1, &smp_capture_depth);
 
+	get_online_cpus_atomic();
 	if (result == 1) {
 		int ncpus = num_online_cpus();
 
@@ -1166,6 +1169,7 @@ void smp_capture(void)
 		printk("done\n");
 #endif
 	}
+	put_online_cpus_atomic();
 }
 
 void smp_release(void)

[PATCH v2 45/45] tile: Use get/put_online_cpus_atomic() to prevent CPU offline

[Srivatsa S. Bhat]

Once stop_machine() is gone from the CPU offline path, we won't be able
to depend on disabling preemption to prevent CPUs from going offline
from under us.

Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

Cc: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/tile/kernel/module.c |    3 +++
 arch/tile/kernel/tlb.c    |   15 +++++++++++++++
 arch/tile/mm/homecache.c  |    3 +++
 3 files changed, 21 insertions(+)

diff --git a/arch/tile/kernel/module.c b/arch/tile/kernel/module.c
index 4918d91..db7d858 100644
--- a/arch/tile/kernel/module.c
+++ b/arch/tile/kernel/module.c
@@ -20,6 +20,7 @@
 #include <linux/fs.h>
 #include <linux/string.h>
 #include <linux/kernel.h>
+#include <linux/cpu.h>
 #include <asm/pgtable.h>
 #include <asm/homecache.h>
 #include <arch/opcode.h>
@@ -79,8 +80,10 @@ void module_free(struct module *mod, void *module_region)
 	vfree(module_region);
 
 	/* Globally flush the L1 icache. */
+	get_online_cpus_atomic();
 	flush_remote(0, HV_FLUSH_EVICT_L1I, cpu_online_mask,
 		     0, 0, 0, NULL, NULL, 0);
+	put_online_cpus_atomic();
 
 	/*
 	 * FIXME: If module_region == mod->module_init, trim exception
diff --git a/arch/tile/kernel/tlb.c b/arch/tile/kernel/tlb.c
index 3fd54d5..a32b9dd 100644
--- a/arch/tile/kernel/tlb.c
+++ b/arch/tile/kernel/tlb.c
@@ -14,6 +14,7 @@
  */
 
 #include <linux/cpumask.h>
+#include <linux/cpu.h>
 #include <linux/module.h>
 #include <linux/hugetlb.h>
 #include <asm/tlbflush.h>
@@ -35,6 +36,8 @@ void flush_tlb_mm(struct mm_struct *mm)
 {
 	HV_Remote_ASID asids[NR_CPUS];
 	int i = 0, cpu;
+
+	get_online_cpus_atomic();
 	for_each_cpu(cpu, mm_cpumask(mm)) {
 		HV_Remote_ASID *asid = &asids[i++];
 		asid->y = cpu / smp_topology.width;
@@ -43,6 +46,7 @@ void flush_tlb_mm(struct mm_struct *mm)
 	}
 	flush_remote(0, HV_FLUSH_EVICT_L1I, mm_cpumask(mm),
 		     0, 0, 0, NULL, asids, i);
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_current_task(void)
@@ -55,8 +59,11 @@ void flush_tlb_page_mm(struct vm_area_struct *vma, struct mm_struct *mm,
 {
 	unsigned long size = vma_kernel_pagesize(vma);
 	int cache = (vma->vm_flags & VM_EXEC) ? HV_FLUSH_EVICT_L1I : 0;
+
+	get_online_cpus_atomic();
 	flush_remote(0, cache, mm_cpumask(mm),
 		     va, size, size, mm_cpumask(mm), NULL, 0);
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_page(struct vm_area_struct *vma, unsigned long va)
@@ -71,13 +78,18 @@ void flush_tlb_range(struct vm_area_struct *vma,
 	unsigned long size = vma_kernel_pagesize(vma);
 	struct mm_struct *mm = vma->vm_mm;
 	int cache = (vma->vm_flags & VM_EXEC) ? HV_FLUSH_EVICT_L1I : 0;
+
+	get_online_cpus_atomic();
 	flush_remote(0, cache, mm_cpumask(mm), start, end - start, size,
 		     mm_cpumask(mm), NULL, 0);
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_all(void)
 {
 	int i;
+
+	get_online_cpus_atomic();
 	for (i = 0; ; ++i) {
 		HV_VirtAddrRange r = hv_inquire_virtual(i);
 		if (r.size == 0)
@@ -89,10 +101,13 @@ void flush_tlb_all(void)
 			     r.start, r.size, HPAGE_SIZE, cpu_online_mask,
 			     NULL, 0);
 	}
+	put_online_cpus_atomic();
 }
 
 void flush_tlb_kernel_range(unsigned long start, unsigned long end)
 {
+	get_online_cpus_atomic();
 	flush_remote(0, HV_FLUSH_EVICT_L1I, cpu_online_mask,
 		     start, end - start, PAGE_SIZE, cpu_online_mask, NULL, 0);
+	put_online_cpus_atomic();
 }
diff --git a/arch/tile/mm/homecache.c b/arch/tile/mm/homecache.c
index 1ae9119..7ff5bf0 100644
--- a/arch/tile/mm/homecache.c
+++ b/arch/tile/mm/homecache.c
@@ -397,9 +397,12 @@ void homecache_change_page_home(struct page *page, int order, int home)
 	BUG_ON(page_count(page) > 1);
 	BUG_ON(page_mapcount(page) != 0);
 	kva = (unsigned long) page_address(page);
+
+	get_online_cpus_atomic();
 	flush_remote(0, HV_FLUSH_EVICT_L2, &cpu_cacheable_map,
 		     kva, pages * PAGE_SIZE, PAGE_SIZE, cpu_online_mask,
 		     NULL, 0);
+	put_online_cpus_atomic();
 
 	for (i = 0; i < pages; ++i, kva += PAGE_SIZE) {
 		pte_t *ptep = virt_to_pte(NULL, kva);