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* [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets
@ 2021-08-07  0:58 Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
                   ` (3 more replies)
  0 siblings, 4 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-07  0:58 UTC (permalink / raw)
  To: linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Hi folks,

this is v2 of:

  https://lore.kernel.org/lkml/20210721115118.729943-1-valentin.schneider@arm.com/

respun from Frederic and Paul's helpful feedback. Tested atop v5.14-rc4-rt6 with
the v1 patches reverted. There, commit 

  d76e0926d835 ("rcu/nocb: Use the rcuog CPU's ->nocb_timer")

prevents the NOCB offload warning from firing if there are no NOCB CPUs (which
is sensible). Adding a single NOCB CPU brings the warning back, which patch 3
fixes.

Note that patches 2 & 4 are already in v5.14-rc4-rt6, but still apply against
mainline.

Revisions
=========

v1 -> v2
++++++++

o Rebased and tested against v5.14-rc4-rt6
o Picked rcutorture patch from
  https://lore.kernel.org/lkml/20210803225437.3612591-2-valentin.schneider@arm.com/
o Added a local_lock to protect NOCB offload state under PREEMPT_RT (Frederic,
  Paul)

Valentin Schneider (4):
  rcutorture: Don't disable softirqs with preemption disabled when
    PREEMPT_RT
  sched: Introduce is_pcpu_safe()
  rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT
  arm64: mm: Make arch_faults_on_old_pte() check for migratability

 arch/arm64/include/asm/pgtable.h |  2 +-
 include/linux/sched.h            | 10 ++++
 kernel/rcu/rcutorture.c          |  2 +
 kernel/rcu/tree.c                |  4 ++
 kernel/rcu/tree.h                |  4 ++
 kernel/rcu/tree_plugin.h         | 82 ++++++++++++++++++++++++++++----
 6 files changed, 94 insertions(+), 10 deletions(-)

--
2.25.1


^ permalink raw reply	[flat|nested] 11+ messages in thread

* [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT
  2021-08-07  0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
@ 2021-08-07  0:58 ` Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
                   ` (2 subsequent siblings)
  3 siblings, 0 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-07  0:58 UTC (permalink / raw)
  To: linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Running RCU torture with CONFIG_PREEMPT_RT under v5.14-rc4-rt6 triggers:

[    8.755472] DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt))
[    8.755482] WARNING: CPU: 1 PID: 137 at kernel/softirq.c:172 __local_bh_disable_ip (kernel/softirq.c:172 (discriminator 31))
[    8.755500] Modules linked in:
[    8.755506] CPU: 1 PID: 137 Comm: rcu_torture_rea Not tainted 5.14.0-rc4-rt6 #171
[    8.755514] Hardware name: ARM Juno development board (r0) (DT)
[    8.755518] pstate: 60000005 (nZCv daif -PAN -UAO -TCO BTYPE=--)
[    8.755622] Call trace:
[    8.755624] __local_bh_disable_ip (kernel/softirq.c:172 (discriminator 31))
[    8.755633] rcutorture_one_extend (kernel/rcu/rcutorture.c:1453)
[    8.755640] rcu_torture_one_read (kernel/rcu/rcutorture.c:1601 kernel/rcu/rcutorture.c:1645)
[    8.755645] rcu_torture_reader (kernel/rcu/rcutorture.c:1737)
[    8.755650] kthread (kernel/kthread.c:327)
[    8.755656] ret_from_fork (arch/arm64/kernel/entry.S:783)
[    8.755663] irq event stamp: 11959
[    8.755666] hardirqs last enabled at (11959): __rcu_read_unlock (kernel/rcu/tree_plugin.h:695 kernel/rcu/tree_plugin.h:451)
[    8.755675] hardirqs last disabled at (11958): __rcu_read_unlock (kernel/rcu/tree_plugin.h:661 kernel/rcu/tree_plugin.h:451)
[    8.755683] softirqs last enabled at (11950): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:261)
[    8.755692] softirqs last disabled at (11942): rcutorture_one_extend (./include/linux/bottom_half.h:19 kernel/rcu/rcutorture.c:1441)

Per this warning, softirqs cannot be disabled in a non-preemptible region
under CONFIG_PREEMPT_RT. Adjust RCU torture accordingly.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/rcu/rcutorture.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index eecd1caef71d..4f3db1d3170d 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1548,6 +1548,8 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
 	 * them on non-RT.
 	 */
 	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		/* Can't disable bh in atomic context under PREEMPT_RT */
+		mask &= ~(RCUTORTURE_RDR_ATOM_BH | RCUTORTURE_RDR_ATOM_RBH);
 		/*
 		 * Can't release the outermost rcu lock in an irq disabled
 		 * section without preemption also being disabled, if irqs
-- 
2.25.1


^ permalink raw reply	[flat|nested] 11+ messages in thread

* [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-07  0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
@ 2021-08-07  0:58 ` Valentin Schneider
  2021-08-07  1:42   ` Mike Galbraith
  2021-08-10  2:42   ` Boqun Feng
  2021-08-07  0:58 ` [PATCH v2 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
  3 siblings, 2 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-07  0:58 UTC (permalink / raw)
  To: linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Some areas use preempt_disable() + preempt_enable() to safely access
per-CPU data. The PREEMPT_RT folks have shown this can also be done by
keeping preemption enabled and instead disabling migration (and acquiring a
sleepable lock, if relevant).

Introduce a helper which checks whether the current task can safely access
per-CPU data, IOW if the task's context guarantees the accesses will target
a single CPU. This accounts for preemption, CPU affinity, and migrate
disable - note that the CPU affinity check also mandates the presence of
PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
upcoming per-CPU access(es) unsafe.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 include/linux/sched.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index debc960f41e3..b77d65f677f6 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void)
 #endif
 }
 
+/* Is the current task guaranteed not to be migrated elsewhere? */
+static inline bool is_pcpu_safe(void)
+{
+#ifdef CONFIG_SMP
+	return !preemptible() || is_percpu_thread() || current->migration_disabled;
+#else
+	return true;
+#endif
+}
+
 /* Per-process atomic flags. */
 #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
 #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
-- 
2.25.1


^ permalink raw reply	[flat|nested] 11+ messages in thread

* [PATCH v2 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT
  2021-08-07  0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
@ 2021-08-07  0:58 ` Valentin Schneider
  2021-08-07  0:58 ` [PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
  3 siblings, 0 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-07  0:58 UTC (permalink / raw)
  To: linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Warning
=======

Running v5.13-rt1 on my arm64 Juno board triggers:

[    0.156302] =============================
[    0.160416] WARNING: suspicious RCU usage
[    0.164529] 5.13.0-rt1 #20 Not tainted
[    0.168300] -----------------------------
[    0.172409] kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state!
[    0.179920]
[    0.179920] other info that might help us debug this:
[    0.179920]
[    0.188037]
[    0.188037] rcu_scheduler_active = 1, debug_locks = 1
[    0.194677] 3 locks held by rcuc/0/11:
[    0.198448] #0: ffff00097ef10cf8 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip (./include/linux/rcupdate.h:662 kernel/softirq.c:171)
[    0.208709] #1: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: rt_spin_lock (kernel/locking/spinlock_rt.c:43 (discriminator 4))
[    0.217134] #2: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: __local_bh_disable_ip (kernel/softirq.c:169)
[    0.226428]
[    0.226428] stack backtrace:
[    0.230889] CPU: 0 PID: 11 Comm: rcuc/0 Not tainted 5.13.0-rt1 #20
[    0.237100] Hardware name: ARM Juno development board (r0) (DT)
[    0.243041] Call trace:
[    0.245497] dump_backtrace (arch/arm64/kernel/stacktrace.c:163)
[    0.249185] show_stack (arch/arm64/kernel/stacktrace.c:219)
[    0.252522] dump_stack (lib/dump_stack.c:122)
[    0.255947] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6439)
[    0.260328] rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58)
[    0.264537] rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777)
[    0.267786] rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876)
[    0.271644] smpboot_thread_fn (kernel/smpboot.c:165 (discriminator 3))
[    0.275767] kthread (kernel/kthread.c:321)
[    0.279013] ret_from_fork (arch/arm64/kernel/entry.S:1005)

In this case, this is the RCU core kthread accessing the local CPU's
rdp. Before that, rcu_cpu_kthread() invokes local_bh_disable().

Under !CONFIG_PREEMPT_RT (and rcutree.use_softirq=0), this ends up
incrementing the preempt_count, which satisfies the "local non-preemptible
read" of rcu_rdp_is_offloaded().

Under CONFIG_PREEMPT_RT however, this becomes

  local_lock(&softirq_ctrl.lock)

which, under the same config, is migrate_disable() + rt_spin_lock(). As
pointed out by Frederic, this is not sufficient to safely access an rdp's
offload state, as the RCU core kthread can be preempted by a kworker
executing rcu_nocb_rdp_offload() [1].

Introduce a local_lock to serialize an rdp's offload state while the rdp's
associated core kthread is executing rcu_core().

rcu_core() preemptability considerations
========================================

As pointed out by Paul [2], keeping rcu_check_quiescent_state() preemptible
(which is the case under CONFIG_PREEMPT_RT) requires some consideration.

note_gp_changes() itself runs with irqs off, and enters
__note_gp_changes() with rnp->lock held (raw_spinlock), thus is safe vs
preemption.

rdp->core_needs_qs *could* change after being read by the RCU core
kthread if it then gets preempted. Consider, with
CONFIG_RCU_STRICT_GRACE_PERIOD:

  rcuc/x                                   task_foo

    rcu_check_quiescent_state()
    `\
      rdp->core_needs_qs == true
			     <PREEMPT>
					   rcu_read_unlock()
					   `\
					     rcu_preempt_deferred_qs_irqrestore()
					     `\
					       rcu_report_qs_rdp()
					       `\
						 rdp->core_needs_qs := false;

This would let rcuc/x's rcu_check_quiescent_state() proceed further down to
rcu_report_qs_rdp(), but if task_foo's earlier rcu_report_qs_rdp()
invocation would have cleared the rdp grpmask from the rnp mask, so
rcuc/x's invocation would simply bail.

Since rcu_report_qs_rdp() can be safely invoked, even if rdp->core_needs_qs
changed, it appears safe to keep rcu_check_quiescent_state() preemptible.

[1]: http://lore.kernel.org/r/20210727230814.GC283787@lothringen
[2]: http://lore.kernel.org/r/20210729010445.GO4397@paulmck-ThinkPad-P17-Gen-1
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/rcu/tree.c        |  4 ++
 kernel/rcu/tree.h        |  4 ++
 kernel/rcu/tree_plugin.h | 82 +++++++++++++++++++++++++++++++++++-----
 3 files changed, 81 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 51f24ecd94b2..caadfec994f3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -87,6 +87,7 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
 	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
 #ifdef CONFIG_RCU_NOCB_CPU
 	.cblist.flags = SEGCBLIST_SOFTIRQ_ONLY,
+	.nocb_local_lock =  INIT_LOCAL_LOCK(nocb_local_lock),
 #endif
 };
 static struct rcu_state rcu_state = {
@@ -2853,10 +2854,12 @@ static void rcu_cpu_kthread(unsigned int cpu)
 {
 	unsigned int *statusp = this_cpu_ptr(&rcu_data.rcu_cpu_kthread_status);
 	char work, *workp = this_cpu_ptr(&rcu_data.rcu_cpu_has_work);
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 	int spincnt;
 
 	trace_rcu_utilization(TPS("Start CPU kthread@rcu_run"));
 	for (spincnt = 0; spincnt < 10; spincnt++) {
+		rcu_nocb_local_lock(rdp);
 		local_bh_disable();
 		*statusp = RCU_KTHREAD_RUNNING;
 		local_irq_disable();
@@ -2866,6 +2869,7 @@ static void rcu_cpu_kthread(unsigned int cpu)
 		if (work)
 			rcu_core();
 		local_bh_enable();
+		rcu_nocb_local_unlock(rdp);
 		if (*workp == 0) {
 			trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
 			*statusp = RCU_KTHREAD_WAITING;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 305cf6aeb408..aa6831255fec 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -210,6 +210,8 @@ struct rcu_data {
 	struct timer_list nocb_timer;	/* Enforce finite deferral. */
 	unsigned long nocb_gp_adv_time;	/* Last call_rcu() CB adv (jiffies). */
 
+	local_lock_t nocb_local_lock;
+
 	/* The following fields are used by call_rcu, hence own cacheline. */
 	raw_spinlock_t nocb_bypass_lock ____cacheline_internodealigned_in_smp;
 	struct rcu_cblist nocb_bypass;	/* Lock-contention-bypass CB list. */
@@ -445,6 +447,8 @@ static void rcu_nocb_unlock(struct rcu_data *rdp);
 static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
 				       unsigned long flags);
 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp);
+static void rcu_nocb_local_lock(struct rcu_data *rdp);
+static void rcu_nocb_local_unlock(struct rcu_data *rdp);
 #ifdef CONFIG_RCU_NOCB_CPU
 static void __init rcu_organize_nocb_kthreads(void);
 #define rcu_nocb_lock_irqsave(rdp, flags)				\
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0ff5e4fb933e..b1487cac9250 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -21,6 +21,17 @@ static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
 	return lockdep_is_held(&rdp->nocb_lock);
 }
 
+static inline int rcu_lockdep_is_held_nocb_local(struct rcu_data *rdp)
+{
+	return lockdep_is_held(
+#ifdef CONFIG_PREEMPT_RT
+		&rdp->nocb_local_lock.lock
+#else
+		&rdp->nocb_local_lock
+#endif
+	);
+}
+
 static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
 {
 	/* Race on early boot between thread creation and assignment */
@@ -38,7 +49,10 @@ static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
 {
 	return 0;
 }
-
+static inline int rcu_lockdep_is_held_nocb_local(struct rcu_data *rdp)
+{
+	return 0;
+}
 static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
 {
 	return false;
@@ -46,23 +60,44 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
 
 #endif /* #ifdef CONFIG_RCU_NOCB_CPU */
 
+/*
+ * Is a local read of the rdp's offloaded state safe and stable?
+ * See rcu_nocb_local_lock() & family.
+ */
+static inline bool rcu_local_offload_access_safe(struct rcu_data *rdp)
+{
+	if (!preemptible())
+		return true;
+
+	if (is_pcpu_safe()) {
+		if (!IS_ENABLED(CONFIG_RCU_NOCB))
+			return true;
+
+		return rcu_lockdep_is_held_nocb_local(rdp);
+	}
+
+	return false;
+}
+
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 {
 	/*
-	 * In order to read the offloaded state of an rdp is a safe
-	 * and stable way and prevent from its value to be changed
-	 * under us, we must either hold the barrier mutex, the cpu
-	 * hotplug lock (read or write) or the nocb lock. Local
-	 * non-preemptible reads are also safe. NOCB kthreads and
-	 * timers have their own means of synchronization against the
-	 * offloaded state updaters.
+	 * In order to read the offloaded state of an rdp is a safe and stable
+	 * way and prevent from its value to be changed under us, we must either...
 	 */
 	RCU_LOCKDEP_WARN(
+		// ...hold the barrier mutex...
 		!(lockdep_is_held(&rcu_state.barrier_mutex) ||
+		  // ... the cpu hotplug lock (read or write)...
 		  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
+		  // ... or the NOCB lock.
 		  rcu_lockdep_is_held_nocb(rdp) ||
+		  // Local reads still require the local state to remain stable
+		  // (preemption disabled / local lock held)
 		  (rdp == this_cpu_ptr(&rcu_data) &&
-		   !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
+		   rcu_local_offload_access_safe(rdp)) ||
+		  // NOCB kthreads and timers have their own means of synchronization
+		  // against the offloaded state updaters.
 		  rcu_current_is_nocb_kthread(rdp)),
 		"Unsafe read of RCU_NOCB offloaded state"
 	);
@@ -1629,6 +1664,22 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
 	}
 }
 
+/*
+ * The invocation of rcu_core() within the RCU core kthreads remains preemptible
+ * under PREEMPT_RT, thus the offload state of a CPU could change while
+ * said kthreads are preempted. Prevent this from happening by protecting the
+ * offload state with a local_lock().
+ */
+static void rcu_nocb_local_lock(struct rcu_data *rdp)
+{
+	local_lock(&rcu_data.nocb_local_lock);
+}
+
+static void rcu_nocb_local_unlock(struct rcu_data *rdp)
+{
+	local_unlock(&rcu_data.nocb_local_lock);
+}
+
 /* Lockdep check that ->cblist may be safely accessed. */
 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
 {
@@ -2396,6 +2447,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp,
 	if (rdp->nocb_cb_sleep)
 		rdp->nocb_cb_sleep = false;
 	rcu_nocb_unlock_irqrestore(rdp, flags);
+	rcu_nocb_local_unlock(rdp);
 
 	/*
 	 * Ignore former value of nocb_cb_sleep and force wake up as it could
@@ -2427,6 +2479,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
 
 	pr_info("De-offloading %d\n", rdp->cpu);
 
+	rcu_nocb_local_lock(rdp);
 	rcu_nocb_lock_irqsave(rdp, flags);
 	/*
 	 * Flush once and for all now. This suffices because we are
@@ -2509,6 +2562,7 @@ static long rcu_nocb_rdp_offload(void *arg)
 	 * Can't use rcu_nocb_lock_irqsave() while we are in
 	 * SEGCBLIST_SOFTIRQ_ONLY mode.
 	 */
+	rcu_nocb_local_lock(rdp);
 	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
 
 	/*
@@ -2868,6 +2922,16 @@ static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
 	local_irq_restore(flags);
 }
 
+/* No ->nocb_local_lock to acquire. */
+static void rcu_nocb_local_lock(struct rcu_data *rdp)
+{
+}
+
+/* No ->nocb_local_lock to release. */
+static void rcu_nocb_local_unlock(struct rcu_data *rdp)
+{
+}
+
 /* Lockdep check that ->cblist may be safely accessed. */
 static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
 {
-- 
2.25.1


^ permalink raw reply	[flat|nested] 11+ messages in thread

* [PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability
  2021-08-07  0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
                   ` (2 preceding siblings ...)
  2021-08-07  0:58 ` [PATCH v2 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
@ 2021-08-07  0:58 ` Valentin Schneider
  3 siblings, 0 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-07  0:58 UTC (permalink / raw)
  To: linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Running v5.13-rt1 on my arm64 Juno board triggers:

[   30.430643] WARNING: CPU: 4 PID: 1 at arch/arm64/include/asm/pgtable.h:985 do_set_pte (./arch/arm64/include/asm/pgtable.h:985 ./arch/arm64/include/asm/pgtable.h:997 mm/memory.c:3830)
[   30.430669] Modules linked in:
[   30.430679] CPU: 4 PID: 1 Comm: init Tainted: G        W         5.13.0-rt1-00002-gcb994ad7c570 #35
[   30.430690] Hardware name: ARM Juno development board (r0) (DT)
[   30.430695] pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
[   30.430874] Call trace:
[   30.430878] do_set_pte (./arch/arm64/include/asm/pgtable.h:985 ./arch/arm64/include/asm/pgtable.h:997 mm/memory.c:3830)
[   30.430886] filemap_map_pages (mm/filemap.c:3222)
[   30.430895] __handle_mm_fault (mm/memory.c:4006 mm/memory.c:4020 mm/memory.c:4153 mm/memory.c:4412 mm/memory.c:4547)
[   30.430904] handle_mm_fault (mm/memory.c:4645)
[   30.430912] do_page_fault (arch/arm64/mm/fault.c:507 arch/arm64/mm/fault.c:607)
[   30.430925] do_translation_fault (arch/arm64/mm/fault.c:692)
[   30.430936] do_mem_abort (arch/arm64/mm/fault.c:821)
[   30.430946] el0_ia (arch/arm64/kernel/entry-common.c:324)
[   30.430959] el0_sync_handler (arch/arm64/kernel/entry-common.c:431)
[   30.430967] el0_sync (arch/arm64/kernel/entry.S:744)
[   30.430977] irq event stamp: 1228384
[   30.430981] hardirqs last enabled at (1228383): lock_page_memcg (mm/memcontrol.c:2005 (discriminator 1))
[   30.430993] hardirqs last disabled at (1228384): el1_dbg (arch/arm64/kernel/entry-common.c:144 arch/arm64/kernel/entry-common.c:234)
[   30.431007] softirqs last enabled at (1228260): __local_bh_enable_ip (./arch/arm64/include/asm/irqflags.h:85 kernel/softirq.c:262)
[   30.431022] softirqs last disabled at (1228232): fpsimd_restore_current_state (./include/linux/bottom_half.h:19 arch/arm64/kernel/fpsimd.c:183 arch/arm64/kernel/fpsimd.c:1182)

CONFIG_PREEMPT_RT turns the PTE lock into a sleepable spinlock. Since
acquiring such a lock also disables migration, any per-CPU access done
under the lock remains safe even if preemptible.

This affects:

  filemap_map_pages()
  `\
    do_set_pte()
    `\
      arch_wants_old_prefaulted_pte()

which checks preemptible() to figure out if the output of
cpu_has_hw_af() (IOW the underlying CPU) will remain stable for the
subsequent operations. Make it use is_pcpu_safe() instead.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 arch/arm64/include/asm/pgtable.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index f09bf5c02891..767a064bedde 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -995,7 +995,7 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
  */
 static inline bool arch_faults_on_old_pte(void)
 {
-	WARN_ON(preemptible());
+	WARN_ON(!is_pcpu_safe());
 
 	return !cpu_has_hw_af();
 }
-- 
2.25.1


^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-07  0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
@ 2021-08-07  1:42   ` Mike Galbraith
  2021-08-08 16:15     ` Valentin Schneider
  2021-08-10  2:42   ` Boqun Feng
  1 sibling, 1 reply; 11+ messages in thread
From: Mike Galbraith @ 2021-08-07  1:42 UTC (permalink / raw)
  To: Valentin Schneider, linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
>
> +static inline bool is_pcpu_safe(void)

Nit: seems odd to avoid spelling it out to save two characters, percpu
is word like, rolls off the ole tongue better than p-c-p-u.

	-Mike


^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-07  1:42   ` Mike Galbraith
@ 2021-08-08 16:15     ` Valentin Schneider
  2021-08-10 12:49       ` Boqun Feng
  0 siblings, 1 reply; 11+ messages in thread
From: Valentin Schneider @ 2021-08-08 16:15 UTC (permalink / raw)
  To: Mike Galbraith, linux-kernel, linux-arm-kernel, rcu, linux-rt-users
  Cc: Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

On 07/08/21 03:42, Mike Galbraith wrote:
> On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
>>
>> +static inline bool is_pcpu_safe(void)
>
> Nit: seems odd to avoid spelling it out to save two characters, percpu
> is word like, rolls off the ole tongue better than p-c-p-u.
>
>       -Mike

True. A quick grep says both versions are used, though "percpu" wins by
about a factor of 2. I'll tweak that for a v3.

^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-07  0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
  2021-08-07  1:42   ` Mike Galbraith
@ 2021-08-10  2:42   ` Boqun Feng
  2021-08-10  9:26     ` Valentin Schneider
  1 sibling, 1 reply; 11+ messages in thread
From: Boqun Feng @ 2021-08-10  2:42 UTC (permalink / raw)
  To: Valentin Schneider
  Cc: linux-kernel, linux-arm-kernel, rcu, linux-rt-users,
	Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Hi,

On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote:
> Some areas use preempt_disable() + preempt_enable() to safely access
> per-CPU data. The PREEMPT_RT folks have shown this can also be done by
> keeping preemption enabled and instead disabling migration (and acquiring a
> sleepable lock, if relevant).
> 
> Introduce a helper which checks whether the current task can safely access
> per-CPU data, IOW if the task's context guarantees the accesses will target
> a single CPU. This accounts for preemption, CPU affinity, and migrate
> disable - note that the CPU affinity check also mandates the presence of
> PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
> upcoming per-CPU access(es) unsafe.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  include/linux/sched.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index debc960f41e3..b77d65f677f6 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void)
>  #endif
>  }
>  
> +/* Is the current task guaranteed not to be migrated elsewhere? */
> +static inline bool is_pcpu_safe(void)
> +{
> +#ifdef CONFIG_SMP
> +	return !preemptible() || is_percpu_thread() || current->migration_disabled;
> +#else
> +	return true;
> +#endif
> +}

I wonder whether the following can happen, say thread A is a worker
thread for CPU 1, so it has the flag PF_NO_SETAFFINITY set.

	{ percpu variable X on CPU 2 is initially 0 }

	thread A
	========

	<preemption enabled>
	if (is_pcpu_safe()) { // nr_cpus_allowed == 1, so return true.
		<preempted>
		<hot unplug CPU 1>
			unbinder_workers(1); // A->cpus_mask becomes cpu_possible_mask
		<back to run on CPU 2>
		__this_cpu_inc(X);
		  tmp = X; // tmp == 0
		  <preempted>
		  <in thread B>
		  this_cpu_inc(X); // X becomes 1
		  <back to run A on CPU 2>
		  X = tmp + 1; // race!
	}

if so, then is_percpu_thread() doesn't indicate is_pcpu_safe()?

Regards,
Boqun

> +
>  /* Per-process atomic flags. */
>  #define PFA_NO_NEW_PRIVS		0	/* May not gain new privileges. */
>  #define PFA_SPREAD_PAGE			1	/* Spread page cache over cpuset */
> -- 
> 2.25.1
> 

^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-10  2:42   ` Boqun Feng
@ 2021-08-10  9:26     ` Valentin Schneider
  0 siblings, 0 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-10  9:26 UTC (permalink / raw)
  To: Boqun Feng
  Cc: linux-kernel, linux-arm-kernel, rcu, linux-rt-users,
	Catalin Marinas, Will Deacon, Ingo Molnar, Peter Zijlstra,
	Thomas Gleixner, Steven Rostedt, Daniel Bristot de Oliveira,
	Sebastian Andrzej Siewior, Paul E. McKenney, Frederic Weisbecker,
	Josh Triplett, Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

Hi,

On 10/08/21 10:42, Boqun Feng wrote:
> Hi,
>
> On Sat, Aug 07, 2021 at 01:58:05AM +0100, Valentin Schneider wrote:
>> Some areas use preempt_disable() + preempt_enable() to safely access
>> per-CPU data. The PREEMPT_RT folks have shown this can also be done by
>> keeping preemption enabled and instead disabling migration (and acquiring a
>> sleepable lock, if relevant).
>> 
>> Introduce a helper which checks whether the current task can safely access
>> per-CPU data, IOW if the task's context guarantees the accesses will target
>> a single CPU. This accounts for preemption, CPU affinity, and migrate
>> disable - note that the CPU affinity check also mandates the presence of
>> PF_NO_SETAFFINITY, as otherwise userspace could concurrently render the
>> upcoming per-CPU access(es) unsafe.
>> 
>> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
>> ---
>>  include/linux/sched.h | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>> 
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index debc960f41e3..b77d65f677f6 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1715,6 +1715,16 @@ static inline bool is_percpu_thread(void)
>>  #endif
>>  }
>>  
>> +/* Is the current task guaranteed not to be migrated elsewhere? */
>> +static inline bool is_pcpu_safe(void)
>> +{
>> +#ifdef CONFIG_SMP
>> +	return !preemptible() || is_percpu_thread() || current->migration_disabled;
>> +#else
>> +	return true;
>> +#endif
>> +}
>
> I wonder whether the following can happen, say thread A is a worker
> thread for CPU 1, so it has the flag PF_NO_SETAFFINITY set.
>
> 	{ percpu variable X on CPU 2 is initially 0 }
>
> 	thread A
> 	========
>
> 	<preemption enabled>
> 	if (is_pcpu_safe()) { // nr_cpus_allowed == 1, so return true.
> 		<preempted>
> 		<hot unplug CPU 1>
> 			unbinder_workers(1); // A->cpus_mask becomes cpu_possible_mask
> 		<back to run on CPU 2>
> 		__this_cpu_inc(X);
> 		  tmp = X; // tmp == 0
> 		  <preempted>
> 		  <in thread B>
> 		  this_cpu_inc(X); // X becomes 1
> 		  <back to run A on CPU 2>
> 		  X = tmp + 1; // race!
> 	}
>
> if so, then is_percpu_thread() doesn't indicate is_pcpu_safe()?
>

You're absolutely right.

migrate_disable() protects the thread against being migrated due to
hotplug, but pure CPU affinity doesn't at all. kthread_is_per_cpu() doesn't
work either, because parking is not the only approach to hotplug for those
(e.g. per-CPU workqueue threads unbind themselves on hotplug, as in your
example).

One could hold cpus_read_lock(), but I don't see much point here. So that
has to be 

  return !preemptible() || current->migration_disabled;

Thanks!

^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-08 16:15     ` Valentin Schneider
@ 2021-08-10 12:49       ` Boqun Feng
  2021-08-10 13:04         ` Valentin Schneider
  0 siblings, 1 reply; 11+ messages in thread
From: Boqun Feng @ 2021-08-10 12:49 UTC (permalink / raw)
  To: Valentin Schneider
  Cc: Mike Galbraith, linux-kernel, linux-arm-kernel, rcu,
	linux-rt-users, Catalin Marinas, Will Deacon, Ingo Molnar,
	Peter Zijlstra, Thomas Gleixner, Steven Rostedt,
	Daniel Bristot de Oliveira, Sebastian Andrzej Siewior,
	Paul E. McKenney, Frederic Weisbecker, Josh Triplett,
	Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

On Sun, Aug 08, 2021 at 05:15:20PM +0100, Valentin Schneider wrote:
> On 07/08/21 03:42, Mike Galbraith wrote:
> > On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
> >>
> >> +static inline bool is_pcpu_safe(void)
> >
> > Nit: seems odd to avoid spelling it out to save two characters, percpu
> > is word like, rolls off the ole tongue better than p-c-p-u.
> >
> >       -Mike
> 
> True. A quick grep says both versions are used, though "percpu" wins by
> about a factor of 2. I'll tweak that for a v3.

I wonder why is_percpu_safe() is the correct name. The safety of
accesses to percpu variables means two things to me:

a)	The thread cannot migrate to other CPU in the middle of
	accessing a percpu variable, in other words, the following
	cannot happen:

	{ percpu variable X is 0 on CPU 0 and 2 on CPU 1
	CPU 0				CPU 1
	========			=========
	<in thread A>
	__this_cpu_inc(X);
	  tmp = X; // tmp is 0
	  <preempted>
	  <migrate to CPU 1>
	  				// continue __this_cpu_inc(X);
					X = tmp + 1; // CPU 0 miss this
						     // increment (this
						     // may be OK), and
						     // CPU 1's X got
						     // corrupted.

b)	The accesses to a percpu variable are exclusive, i.e. no
	interrupt or preemption can happen in the middle of accessing,
	in other words, the following cannot happen:

	{ percpu variable X is 0 on CPU 0 }
	CPU 0
	========
	<in thread A>
	__this_cpu_inc(X);
	  tmp = X; // tmp is 0
	  <preempted>
	  <in other thread>
	  this_cpu_inc(X); // X is 1 afterwards.
	  <back to thread A>
	  X = tmp + 1; // X is 1, and we have a race condition.

And the is_p{er}cpu_safe() only detects the first, and it doesn't mean
totally safe for percpu accesses.

Maybe we can implement a migratable()? Although not sure it's a English
word.

Regards,
Boqun


^ permalink raw reply	[flat|nested] 11+ messages in thread

* Re: [PATCH v2 2/4] sched: Introduce is_pcpu_safe()
  2021-08-10 12:49       ` Boqun Feng
@ 2021-08-10 13:04         ` Valentin Schneider
  0 siblings, 0 replies; 11+ messages in thread
From: Valentin Schneider @ 2021-08-10 13:04 UTC (permalink / raw)
  To: Boqun Feng
  Cc: Mike Galbraith, linux-kernel, linux-arm-kernel, rcu,
	linux-rt-users, Catalin Marinas, Will Deacon, Ingo Molnar,
	Peter Zijlstra, Thomas Gleixner, Steven Rostedt,
	Daniel Bristot de Oliveira, Sebastian Andrzej Siewior,
	Paul E. McKenney, Frederic Weisbecker, Josh Triplett,
	Mathieu Desnoyers, Davidlohr Bueso, Lai Jiangshan,
	Joel Fernandes, Anshuman Khandual, Vincenzo Frascino,
	Steven Price, Ard Biesheuvel

On 10/08/21 20:49, Boqun Feng wrote:
> On Sun, Aug 08, 2021 at 05:15:20PM +0100, Valentin Schneider wrote:
>> On 07/08/21 03:42, Mike Galbraith wrote:
>> > On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
>> >>
>> >> +static inline bool is_pcpu_safe(void)
>> >
>> > Nit: seems odd to avoid spelling it out to save two characters, percpu
>> > is word like, rolls off the ole tongue better than p-c-p-u.
>> >
>> >       -Mike
>> 
>> True. A quick grep says both versions are used, though "percpu" wins by
>> about a factor of 2. I'll tweak that for a v3.
>
> I wonder why is_percpu_safe() is the correct name. The safety of
> accesses to percpu variables means two things to me:
>
> a)	The thread cannot migrate to other CPU in the middle of
> 	accessing a percpu variable, in other words, the following
> 	cannot happen:
>
> 	{ percpu variable X is 0 on CPU 0 and 2 on CPU 1
> 	CPU 0				CPU 1
> 	========			=========
> 	<in thread A>
> 	__this_cpu_inc(X);
> 	  tmp = X; // tmp is 0
> 	  <preempted>
> 	  <migrate to CPU 1>
> 	  				// continue __this_cpu_inc(X);
> 					X = tmp + 1; // CPU 0 miss this
> 						     // increment (this
> 						     // may be OK), and
> 						     // CPU 1's X got
> 						     // corrupted.
>
> b)	The accesses to a percpu variable are exclusive, i.e. no
> 	interrupt or preemption can happen in the middle of accessing,
> 	in other words, the following cannot happen:
>
> 	{ percpu variable X is 0 on CPU 0 }
> 	CPU 0
> 	========
> 	<in thread A>
> 	__this_cpu_inc(X);
> 	  tmp = X; // tmp is 0
> 	  <preempted>
> 	  <in other thread>
> 	  this_cpu_inc(X); // X is 1 afterwards.
> 	  <back to thread A>
> 	  X = tmp + 1; // X is 1, and we have a race condition.
>
> And the is_p{er}cpu_safe() only detects the first, and it doesn't mean
> totally safe for percpu accesses.
>

Right. I do briefly point this out in the changelog (the bit about
"acquiring a sleepable lock if relevant"), but that doesn't do much to
clarify the helper name itself.

> Maybe we can implement a migratable()? Although not sure it's a English
> word.
>

Funnily enough that is exactly how I named the thing in my initial draft,
but then I somehow convinced myself that tailoring the name to per-CPU
accesses would make its intent clearer.

I think you're right that "migratable()" is less confusing at the end of
the day. Oh well, so much for overthinking the naming problem :-)

> Regards,
> Boqun

^ permalink raw reply	[flat|nested] 11+ messages in thread

end of thread, other threads:[~2021-08-10 13:04 UTC | newest]

Thread overview: 11+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-07  0:58 [PATCH v2 0/4] rcu, arm64: PREEMPT_RT fixlets Valentin Schneider
2021-08-07  0:58 ` [PATCH v2 1/4] rcutorture: Don't disable softirqs with preemption disabled when PREEMPT_RT Valentin Schneider
2021-08-07  0:58 ` [PATCH v2 2/4] sched: Introduce is_pcpu_safe() Valentin Schneider
2021-08-07  1:42   ` Mike Galbraith
2021-08-08 16:15     ` Valentin Schneider
2021-08-10 12:49       ` Boqun Feng
2021-08-10 13:04         ` Valentin Schneider
2021-08-10  2:42   ` Boqun Feng
2021-08-10  9:26     ` Valentin Schneider
2021-08-07  0:58 ` [PATCH v2 3/4] rcu/nocb: Protect NOCB state via local_lock() under PREEMPT_RT Valentin Schneider
2021-08-07  0:58 ` [PATCH v2 4/4] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider

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