[PATCH tip/core/rcu 0/6] SRCU updates for v5.14

[PATCH tip/core/rcu 0/6] SRCU updates for v5.14

[Paul E. McKenney]

Hello!

This series provides some updates for SRCU, all courtesy of Frederic
Weisbecker.

1.	Remove superfluous sdp->srcu_lock_count zero filling.

2.	Remove superfluous ssp initialization for early callbacks.

3.	Unconditionally embed struct lockdep_map.

4.	Initialize SRCU after timers.

5.	Fix broken node geometry after early ssp init.

6.	Early test SRCU polling start.

						Thanx, Paul

------------------------------------------------------------------------

 b/include/linux/srcu.h     |    6 ++++++
 b/include/linux/srcutree.h |    2 --
 b/init/main.c              |    2 ++
 b/kernel/rcu/rcu.h         |    6 ------
 b/kernel/rcu/srcutree.c    |   12 ++----------
 b/kernel/rcu/tiny.c        |    1 -
 b/kernel/rcu/tree.c        |    1 -
 b/kernel/rcu/update.c      |    6 +++++-
 kernel/rcu/rcu.h           |    2 ++
 kernel/rcu/srcutree.c      |    9 ++++++++-
 kernel/rcu/tree.c          |   16 +++++++++++++++-
 11 files changed, 40 insertions(+), 23 deletions(-)

[PATCH tip/core/rcu 1/6] srcu: Remove superfluous sdp->srcu_lock_count zero filling

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

Because alloc_percpu() zeroes out the allocated memory, there is no need
to zero-fill newly allocated per-CPU memory.  This commit therefore removes
the loop zeroing the ->srcu_lock_count and ->srcu_unlock_count arrays
from init_srcu_struct_nodes().  This is the only use of that function's
is_static parameter, which this commit also removes.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/srcutree.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index e26547b34ad3..3414aff2a4b0 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -80,7 +80,7 @@ do {									\
  * srcu_read_unlock() running against them.  So if the is_static parameter
  * is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
  */
-static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
+static void init_srcu_struct_nodes(struct srcu_struct *ssp)
 {
 	int cpu;
 	int i;
@@ -148,14 +148,6 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static)
 		timer_setup(&sdp->delay_work, srcu_delay_timer, 0);
 		sdp->ssp = ssp;
 		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
-		if (is_static)
-			continue;
-
-		/* Dynamically allocated, better be no srcu_read_locks()! */
-		for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
-			sdp->srcu_lock_count[i] = 0;
-			sdp->srcu_unlock_count[i] = 0;
-		}
 	}
 }
 
@@ -179,7 +171,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 		ssp->sda = alloc_percpu(struct srcu_data);
 	if (!ssp->sda)
 		return -ENOMEM;
-	init_srcu_struct_nodes(ssp, is_static);
+	init_srcu_struct_nodes(ssp);
 	ssp->srcu_gp_seq_needed_exp = 0;
 	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 2/6] srcu: Remove superfluous ssp initialization for early callbacks

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

Pre-srcu_init() invocations of call_srcu() initialize the srcu_struct
structure in question, so there is no need to check this initialization
in srcu_init() when initiating grace periods for srcu_struct structures
that had early call_srcu() invocations.  This commit therefore drops
the calls to check_init_srcu_struct() in srcu_init().

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/srcutree.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 3414aff2a4b0..f4f0cbf7a02b 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1388,7 +1388,6 @@ void __init srcu_init(void)
 	while (!list_empty(&srcu_boot_list)) {
 		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
 				      work.work.entry);
-		check_init_srcu_struct(ssp);
 		list_del_init(&ssp->work.work.entry);
 		queue_work(rcu_gp_wq, &ssp->work.work);
 	}
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 3/6] srcu: Unconditionally embed struct lockdep_map

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

Since struct lockdep_map has zero size when CONFIG_DEBUG_LOCK_ALLOC=n,
this commit removes the #ifdef from the srcu_struct structure's ->dep_map.
This change will simplify further manipulations of this field.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/srcutree.h | 2 --
 1 file changed, 2 deletions(-)

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index 9cfcc8a756ae..cb1f4351e8ba 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -82,9 +82,7 @@ struct srcu_struct {
 						/*  callback for the barrier */
 						/*  operation. */
 	struct delayed_work work;
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
-#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 };
 
 /* Values for state variable (bottom bits of ->srcu_gp_seq). */
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 4/6] srcu: Initialize SRCU after timers

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

Once srcu_init() is called, the SRCU core will make use of delayed
workqueues, which rely on timers.  However init_timers() is called
several steps after rcu_init().  This means that a call_srcu() after
rcu_init() but before init_timers() would find itself within a dangerously
uninitialized timer core.

This commit therefore creates a separate call to srcu_init() after
init_timer() completes, which ensures that we stay in early SRCU mode
until timers are safe(r).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/srcu.h  | 6 ++++++
 init/main.c           | 2 ++
 kernel/rcu/rcu.h      | 6 ------
 kernel/rcu/srcutree.c | 5 +++++
 kernel/rcu/tiny.c     | 1 -
 kernel/rcu/tree.c     | 1 -
 6 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index a0895bbf71ce..e6011a9975af 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -64,6 +64,12 @@ unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
 unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
 bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
 
+#ifdef CONFIG_SRCU
+void srcu_init(void);
+#else /* #ifdef CONFIG_SRCU */
+static inline void srcu_init(void) { }
+#endif /* #else #ifdef CONFIG_SRCU */
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 /**
diff --git a/init/main.c b/init/main.c
index eb01e121d2f1..7b6f49c4d388 100644
--- a/init/main.c
+++ b/init/main.c
@@ -42,6 +42,7 @@
 #include <linux/profile.h>
 #include <linux/kfence.h>
 #include <linux/rcupdate.h>
+#include <linux/srcu.h>
 #include <linux/moduleparam.h>
 #include <linux/kallsyms.h>
 #include <linux/writeback.h>
@@ -979,6 +980,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
 	tick_init();
 	rcu_init_nohz();
 	init_timers();
+	srcu_init();
 	hrtimers_init();
 	softirq_init();
 	timekeeping_init();
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index bf0827d4b659..ca3f2af32bf8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -422,12 +422,6 @@ do {									\
 
 #endif /* #if defined(CONFIG_SRCU) || !defined(CONFIG_TINY_RCU) */
 
-#ifdef CONFIG_SRCU
-void srcu_init(void);
-#else /* #ifdef CONFIG_SRCU */
-static inline void srcu_init(void) { }
-#endif /* #else #ifdef CONFIG_SRCU */
-
 #ifdef CONFIG_TINY_RCU
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
 static inline bool rcu_gp_is_normal(void) { return true; }
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index f4f0cbf7a02b..9ec35c158740 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1384,6 +1384,11 @@ void __init srcu_init(void)
 {
 	struct srcu_struct *ssp;
 
+	/*
+	 * Once that is set, call_srcu() can follow the normal path and
+	 * queue delayed work. This must follow RCU workqueues creation
+	 * and timers initialization.
+	 */
 	srcu_init_done = true;
 	while (!list_empty(&srcu_boot_list)) {
 		ssp = list_first_entry(&srcu_boot_list, struct srcu_struct,
diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
index c8a029fbb114..340b3f8b090d 100644
--- a/kernel/rcu/tiny.c
+++ b/kernel/rcu/tiny.c
@@ -221,5 +221,4 @@ void __init rcu_init(void)
 {
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 	rcu_early_boot_tests();
-	srcu_init();
 }
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e78b2430c16..c35b15229cff 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4735,7 +4735,6 @@ void __init rcu_init(void)
 	WARN_ON(!rcu_gp_wq);
 	rcu_par_gp_wq = alloc_workqueue("rcu_par_gp", WQ_MEM_RECLAIM, 0);
 	WARN_ON(!rcu_par_gp_wq);
-	srcu_init();
 
 	/* Fill in default value for rcutree.qovld boot parameter. */
 	/* -After- the rcu_node ->lock fields are initialized! */
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 5/6] srcu: Fix broken node geometry after early ssp init

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

An srcu_struct structure that is initialized before rcu_init_geometry()
will have its srcu_node hierarchy based on CONFIG_NR_CPUS.  Once
rcu_init_geometry() is called, this hierarchy is compressed as needed
for the actual maximum number of CPUs for this system.

Later on, that srcu_struct structure is confused, sometimes referring
to its initial CONFIG_NR_CPUS-based hierarchy, and sometimes instead
to the new num_possible_cpus() hierarchy.  For example, each of its
->mynode fields continues to reference the original leaf rcu_node
structures, some of which might no longer exist.  On the other hand,
srcu_for_each_node_breadth_first() traverses to the new node hierarchy.

There are at least two bad possible outcomes to this:

1) a) A callback enqueued early on an srcu_data structure (call it
      *sdp) is recorded pending on sdp->mynode->srcu_data_have_cbs in
      srcu_funnel_gp_start() with sdp->mynode pointing to a deep leaf
      (say 3 levels).

   b) The grace period ends after rcu_init_geometry() shrinks the
      nodes level to a single one.  srcu_gp_end() walks through the new
      srcu_node hierarchy without ever reaching the old leaves so the
      callback is never executed.

   This is easily reproduced on an 8 CPUs machine with CONFIG_NR_CPUS >= 32
   and "rcupdate.rcu_self_test=1". The srcu_barrier() after early tests
   verification never completes and the boot hangs:

	[ 5413.141029] INFO: task swapper/0:1 blocked for more than 4915 seconds.
	[ 5413.147564]       Not tainted 5.12.0-rc4+ #28
	[ 5413.151927] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
	[ 5413.159753] task:swapper/0       state:D stack:    0 pid:    1 ppid:     0 flags:0x00004000
	[ 5413.168099] Call Trace:
	[ 5413.170555]  __schedule+0x36c/0x930
	[ 5413.174057]  ? wait_for_completion+0x88/0x110
	[ 5413.178423]  schedule+0x46/0xf0
	[ 5413.181575]  schedule_timeout+0x284/0x380
	[ 5413.185591]  ? wait_for_completion+0x88/0x110
	[ 5413.189957]  ? mark_held_locks+0x61/0x80
	[ 5413.193882]  ? mark_held_locks+0x61/0x80
	[ 5413.197809]  ? _raw_spin_unlock_irq+0x24/0x50
	[ 5413.202173]  ? wait_for_completion+0x88/0x110
	[ 5413.206535]  wait_for_completion+0xb4/0x110
	[ 5413.210724]  ? srcu_torture_stats_print+0x110/0x110
	[ 5413.215610]  srcu_barrier+0x187/0x200
	[ 5413.219277]  ? rcu_tasks_verify_self_tests+0x50/0x50
	[ 5413.224244]  ? rdinit_setup+0x2b/0x2b
	[ 5413.227907]  rcu_verify_early_boot_tests+0x2d/0x40
	[ 5413.232700]  do_one_initcall+0x63/0x310
	[ 5413.236541]  ? rdinit_setup+0x2b/0x2b
	[ 5413.240207]  ? rcu_read_lock_sched_held+0x52/0x80
	[ 5413.244912]  kernel_init_freeable+0x253/0x28f
	[ 5413.249273]  ? rest_init+0x250/0x250
	[ 5413.252846]  kernel_init+0xa/0x110
	[ 5413.256257]  ret_from_fork+0x22/0x30

2) An srcu_struct structure that is initialized before rcu_init_geometry()
   and used afterward will always have stale rdp->mynode references,
   resulting in callbacks to be missed in srcu_gp_end(), just like in
   the previous scenario.

This commit therefore causes init_srcu_struct_nodes to initialize the
geometry, if needed.  This ensures that the srcu_node hierarchy is
properly built and distributed from the get-go.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/rcu.h      |  2 ++
 kernel/rcu/srcutree.c |  3 +++
 kernel/rcu/tree.c     | 16 +++++++++++++++-
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index ca3f2af32bf8..28de768bd875 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -308,6 +308,8 @@ static inline void rcu_init_levelspread(int *levelspread, const int *levelcnt)
 	}
 }
 
+extern void rcu_init_geometry(void);
+
 /* Returns a pointer to the first leaf rcu_node structure. */
 #define rcu_first_leaf_node() (rcu_state.level[rcu_num_lvls - 1])
 
diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 9ec35c158740..5fdbde687feb 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -90,6 +90,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp)
 	struct srcu_node *snp;
 	struct srcu_node *snp_first;
 
+	/* Initialize geometry if it has not already been initialized. */
+	rcu_init_geometry();
+
 	/* Work out the overall tree geometry. */
 	ssp->level[0] = &ssp->node[0];
 	for (i = 1; i < rcu_num_lvls; i++)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c35b15229cff..0420b23fc9d0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4582,11 +4582,25 @@ static void __init rcu_init_one(void)
  * replace the definitions in tree.h because those are needed to size
  * the ->node array in the rcu_state structure.
  */
-static void __init rcu_init_geometry(void)
+void rcu_init_geometry(void)
 {
 	ulong d;
 	int i;
+	static unsigned long old_nr_cpu_ids;
 	int rcu_capacity[RCU_NUM_LVLS];
+	static bool initialized;
+
+	if (initialized) {
+		/*
+		 * Warn if setup_nr_cpu_ids() had not yet been invoked,
+		 * unless nr_cpus_ids == NR_CPUS, in which case who cares?
+		 */
+		WARN_ON_ONCE(old_nr_cpu_ids != nr_cpu_ids);
+		return;
+	}
+
+	old_nr_cpu_ids = nr_cpu_ids;
+	initialized = true;
 
 	/*
 	 * Initialize any unspecified boot parameters.
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 6/6] srcu: Early test SRCU polling start

[Paul E. McKenney]

From: Frederic Weisbecker <frederic@kernel.org>

Place an early call to start_poll_synchronize_srcu() before the the
invocation of call_srcu() on the same srcu_struct structure.

After the later call to srcu_barrier(), the completion of the
first grace period should be visible to a subsequent invocation of
poll_state_synchronize_srcu(), and if not, warn.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/update.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index b95ae86c40a7..0aa118ac37ba 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -524,6 +524,7 @@ static void test_callback(struct rcu_head *r)
 }
 
 DEFINE_STATIC_SRCU(early_srcu);
+static unsigned long early_srcu_cookie;
 
 struct early_boot_kfree_rcu {
 	struct rcu_head rh;
@@ -536,8 +537,10 @@ static void early_boot_test_call_rcu(void)
 	struct early_boot_kfree_rcu *rhp;
 
 	call_rcu(&head, test_callback);
-	if (IS_ENABLED(CONFIG_SRCU))
+	if (IS_ENABLED(CONFIG_SRCU)) {
+		early_srcu_cookie = start_poll_synchronize_srcu(&early_srcu);
 		call_srcu(&early_srcu, &shead, test_callback);
+	}
 	rhp = kmalloc(sizeof(*rhp), GFP_KERNEL);
 	if (!WARN_ON_ONCE(!rhp))
 		kfree_rcu(rhp, rh);
@@ -563,6 +566,7 @@ static int rcu_verify_early_boot_tests(void)
 		if (IS_ENABLED(CONFIG_SRCU)) {
 			early_boot_test_counter++;
 			srcu_barrier(&early_srcu);
+			WARN_ON_ONCE(!poll_state_synchronize_srcu(&early_srcu, early_srcu_cookie));
 		}
 	}
 	if (rcu_self_test_counter != early_boot_test_counter) {
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 1/4] rcu-tasks: Add block comment laying out RCU Tasks design

[Paul E. McKenney]

This commit adds a block comment that gives a high-level overview of how
RCU tasks grace periods progress.  It also adds a note about how exiting
tasks are handled, plus it gives an overview of the memory ordering.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 350ebf5051f9..94d2c2c7f0ab 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -377,6 +377,46 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 // Finally, this implementation does not support high call_rcu_tasks()
 // rates from multiple CPUs.  If this is required, per-CPU callback lists
 // will be needed.
+//
+// The implementation uses rcu_tasks_wait_gp(), which relies on function
+// pointers in the rcu_tasks structure.  The rcu_spawn_tasks_kthread()
+// function sets these function pointers up so that rcu_tasks_wait_gp()
+// invokes these functions in this order:
+//
+// rcu_tasks_pregp_step():
+//	Invokes synchronize_rcu() in order to wait for all in-flight
+//	t->on_rq and t->nvcsw transitions to complete.	This works because
+//	all such transitions are carried out with interrupts disabled.
+// rcu_tasks_pertask(), invoked on every non-idle task:
+//	For every runnable non-idle task other than the current one, use
+//	get_task_struct() to pin down that task, snapshot that task's
+//	number of voluntary context switches, and add that task to the
+//	holdout list.
+// rcu_tasks_postscan():
+//	Invoke synchronize_srcu() to ensure that all tasks that were
+//	in the process of exiting (and which thus might not know to
+//	synchronize with this RCU Tasks grace period) have completed
+//	exiting.
+// check_all_holdout_tasks(), repeatedly until holdout list is empty:
+//	Scans the holdout list, attempting to identify a quiescent state
+//	for each task on the list.  If there is a quiescent state, the
+//	corresponding task is removed from the holdout list.
+// rcu_tasks_postgp():
+//	Invokes synchronize_rcu() in order to ensure that all prior
+//	t->on_rq and t->nvcsw transitions are seen by all CPUs and tasks
+//	to have happened before the end of this RCU Tasks grace period.
+//	Again, this works because all such transitions are carried out
+//	with interrupts disabled.
+//
+// For each exiting task, the exit_tasks_rcu_start() and
+// exit_tasks_rcu_finish() functions begin and end, respectively, the SRCU
+// read-side critical sections waited for by rcu_tasks_postscan().
+//
+// Pre-grace-period update-side code is ordered before the grace via the
+// ->cbs_lock and the smp_mb__after_spinlock().  Pre-grace-period read-side
+// code is ordered before the grace period via synchronize_rcu() call
+// in rcu_tasks_pregp_step() and by the scheduler's locks and interrupt
+// disabling.
 
 /* Pre-grace-period preparation. */
 static void rcu_tasks_pregp_step(void)
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 2/4] rcu-tasks: Add block comment laying out RCU Rude design

[Paul E. McKenney]

This commit adds a block comment that gives a high-level overview of
how RCU Rude grace periods progress.  It also gives an overview of the
memory ordering.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 94d2c2c7f0ab..d6aa352cd705 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -645,8 +645,13 @@ void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
 // passing an empty function to schedule_on_each_cpu().  This approach
 // provides an asynchronous call_rcu_tasks_rude() API and batching
 // of concurrent calls to the synchronous synchronize_rcu_rude() API.
-// This sends IPIs far and wide and induces otherwise unnecessary context
-// switches on all online CPUs, whether idle or not.
+// This invokes schedule_on_each_cpu() in order to send IPIs far and wide
+// and induces otherwise unnecessary context switches on all online CPUs,
+// whether idle or not.
+//
+// Callback handling is provided by the rcu_tasks_kthread() function.
+//
+// Ordering is provided by the scheduler's context-switch code.
 
 // Empty function to allow workqueues to force a context switch.
 static void rcu_tasks_be_rude(struct work_struct *work)
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states

[Paul E. McKenney]

Heavy networking load can cause a CPU to execute continuously and
indefinitely within ksoftirqd, in which case there will be no voluntary
task switches and thus no RCU-tasks quiescent states.  This commit
therefore causes the exiting rcu_softirq_qs() to provide an RCU-tasks
quiescent state.

This of course means that __do_softirq() and its callers cannot be
invoked from within a tracing trampoline.

Reported-by: Toke Høiland-Jørgensen <toke@redhat.com>
Tested-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
---
 kernel/rcu/tree.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e78b2430c16..f4daa4e60b14 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -242,6 +242,7 @@ void rcu_softirq_qs(void)
 {
 	rcu_qs();
 	rcu_preempt_deferred_qs(current);
+	rcu_tasks_qs(current, false);
 }
 
 /*
-- 
2.31.1.189.g2e36527f23

[PATCH tip/core/rcu 4/4] tasks-rcu: Make show_rcu_tasks_gp_kthreads() be static inline

[Paul E. McKenney]

In some architectures, the no-op variant of show_rcu_tasks_gp_kthreads()
get "no previous prototype" compiler warnings.  These are false positives
given that kernel/rcu/tasks.h is included only once.  But why put up
with the compiler noise?

This commit therefore adds "static inline" to this definition to force
the compiler to accept this situation, while also moving it to its proper
place in kernel/rcu/rcu.h.

Reported-by: kernel test robot <lkp@intel.com>
[ paulmck: Update per Stephen Rothwell feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/rcu.h   | 4 ++++
 kernel/rcu/tasks.h | 1 -
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index bf0827d4b659..c0b3ab6736b8 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -441,7 +441,11 @@ bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
 void rcu_expedite_gp(void);
 void rcu_unexpedite_gp(void);
 void rcupdate_announce_bootup_oddness(void);
+#ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);
+#else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+static inline void show_rcu_tasks_gp_kthreads(void) {}
+#endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 void rcu_request_urgent_qs_task(struct task_struct *t);
 #endif /* #else #ifdef CONFIG_TINY_RCU */
 
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d6aa352cd705..fc218539d151 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1401,5 +1401,4 @@ void __init rcu_init_tasks_generic(void)
 
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 static inline void rcu_tasks_bootup_oddness(void) {}
-void show_rcu_tasks_gp_kthreads(void) {}
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
-- 
2.31.1.189.g2e36527f23