[PATCH AUTOSEL 6.7 02/11] rcu-tasks: Repair RCU Tasks Trace quiescence check

[PATCH AUTOSEL 6.7 02/11] rcu-tasks: Repair RCU Tasks Trace quiescence check

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 2eb52fa8900e642b3b5054c4bf9776089d2a935f ]

The context-switch-time check for RCU Tasks Trace quiescence expects
current->trc_reader_special.b.need_qs to be zero, and if so, updates
it to TRC_NEED_QS_CHECKED.  This is backwards, because if this value
is zero, there is no RCU Tasks Trace grace period in flight, an thus
no need for a quiescent state.  Instead, when a grace period starts,
this field is set to TRC_NEED_QS.

This commit therefore changes the check from zero to TRC_NEED_QS.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/rcupdate.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 31d523c4e0893..457694a274bcf 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -187,9 +187,9 @@ void rcu_tasks_trace_qs_blkd(struct task_struct *t);
 	do {									\
 		int ___rttq_nesting = READ_ONCE((t)->trc_reader_nesting);	\
 										\
-		if (likely(!READ_ONCE((t)->trc_reader_special.b.need_qs)) &&	\
+		if (unlikely(READ_ONCE((t)->trc_reader_special.b.need_qs) == TRC_NEED_QS) &&	\
 		    likely(!___rttq_nesting)) {					\
-			rcu_trc_cmpxchg_need_qs((t), 0,	TRC_NEED_QS_CHECKED);	\
+			rcu_trc_cmpxchg_need_qs((t), TRC_NEED_QS, TRC_NEED_QS_CHECKED);	\
 		} else if (___rttq_nesting && ___rttq_nesting != INT_MIN &&	\
 			   !READ_ONCE((t)->trc_reader_special.b.blocked)) {	\
 			rcu_tasks_trace_qs_blkd(t);				\
-- 
2.43.0

[PATCH AUTOSEL 6.7 03/11] rcu-tasks: Add data to eliminate RCU-tasks/do_exit() deadlocks

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit bfe93930ea1ea3c6c115a7d44af6e4fea609067e ]

Holding a mutex across synchronize_rcu_tasks() and acquiring
that same mutex in code called from do_exit() after its call to
exit_tasks_rcu_start() but before its call to exit_tasks_rcu_stop()
results in deadlock.  This is by design, because tasks that are far
enough into do_exit() are no longer present on the tasks list, making
it a bit difficult for RCU Tasks to find them, let alone wait on them
to do a voluntary context switch.  However, such deadlocks are becoming
more frequent.  In addition, lockdep currently does not detect such
deadlocks and they can be difficult to reproduce.

In addition, if a task voluntarily context switches during that time
(for example, if it blocks acquiring a mutex), then this task is in an
RCU Tasks quiescent state.  And with some adjustments, RCU Tasks could
just as well take advantage of that fact.

This commit therefore adds the data structures that will be needed
to rely on these quiescent states and to eliminate these deadlocks.

Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@huawei.com/

Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/sched.h | 2 ++
 kernel/rcu/tasks.h    | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 292c316972485..c610cba18e0e2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -855,6 +855,8 @@ struct task_struct {
 	u8				rcu_tasks_idx;
 	int				rcu_tasks_idle_cpu;
 	struct list_head		rcu_tasks_holdout_list;
+	int				rcu_tasks_exit_cpu;
+	struct list_head		rcu_tasks_exit_list;
 #endif /* #ifdef CONFIG_TASKS_RCU */
 
 #ifdef CONFIG_TASKS_TRACE_RCU
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index f54d5782eca0b..3580055342ca9 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -32,6 +32,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
  * @rtp_irq_work: IRQ work queue for deferred wakeups.
  * @barrier_q_head: RCU callback for barrier operation.
  * @rtp_blkd_tasks: List of tasks blocked as readers.
+ * @rtp_exit_list: List of tasks in the latter portion of do_exit().
  * @cpu: CPU number corresponding to this entry.
  * @rtpp: Pointer to the rcu_tasks structure.
  */
@@ -46,6 +47,7 @@ struct rcu_tasks_percpu {
 	struct irq_work rtp_irq_work;
 	struct rcu_head barrier_q_head;
 	struct list_head rtp_blkd_tasks;
+	struct list_head rtp_exit_list;
 	int cpu;
 	struct rcu_tasks *rtpp;
 };
-- 
2.43.0

[PATCH AUTOSEL 6.7 04/11] rcu-tasks: Initialize data to eliminate RCU-tasks/do_exit() deadlocks

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 46faf9d8e1d52e4a91c382c6c72da6bd8e68297b ]

Holding a mutex across synchronize_rcu_tasks() and acquiring
that same mutex in code called from do_exit() after its call to
exit_tasks_rcu_start() but before its call to exit_tasks_rcu_stop()
results in deadlock.  This is by design, because tasks that are far
enough into do_exit() are no longer present on the tasks list, making
it a bit difficult for RCU Tasks to find them, let alone wait on them
to do a voluntary context switch.  However, such deadlocks are becoming
more frequent.  In addition, lockdep currently does not detect such
deadlocks and they can be difficult to reproduce.

In addition, if a task voluntarily context switches during that time
(for example, if it blocks acquiring a mutex), then this task is in an
RCU Tasks quiescent state.  And with some adjustments, RCU Tasks could
just as well take advantage of that fact.

This commit therefore initializes the data structures that will be needed
to rely on these quiescent states and to eliminate these deadlocks.

Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@huawei.com/

Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 init/init_task.c   | 1 +
 kernel/fork.c      | 1 +
 kernel/rcu/tasks.h | 2 ++
 3 files changed, 4 insertions(+)

diff --git a/init/init_task.c b/init/init_task.c
index 5727d42149c33..65f037bff457a 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -152,6 +152,7 @@ struct task_struct init_task
 	.rcu_tasks_holdout = false,
 	.rcu_tasks_holdout_list = LIST_HEAD_INIT(init_task.rcu_tasks_holdout_list),
 	.rcu_tasks_idle_cpu = -1,
+	.rcu_tasks_exit_list = LIST_HEAD_INIT(init_task.rcu_tasks_exit_list),
 #endif
 #ifdef CONFIG_TASKS_TRACE_RCU
 	.trc_reader_nesting = 0,
diff --git a/kernel/fork.c b/kernel/fork.c
index 10917c3e1f036..6bacd515e0eb5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1981,6 +1981,7 @@ static inline void rcu_copy_process(struct task_struct *p)
 	p->rcu_tasks_holdout = false;
 	INIT_LIST_HEAD(&p->rcu_tasks_holdout_list);
 	p->rcu_tasks_idle_cpu = -1;
+	INIT_LIST_HEAD(&p->rcu_tasks_exit_list);
 #endif /* #ifdef CONFIG_TASKS_RCU */
 #ifdef CONFIG_TASKS_TRACE_RCU
 	p->trc_reader_nesting = 0;
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 3580055342ca9..ac8eb667a6707 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -277,6 +277,8 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
 		rtpcp->rtpp = rtp;
 		if (!rtpcp->rtp_blkd_tasks.next)
 			INIT_LIST_HEAD(&rtpcp->rtp_blkd_tasks);
+		if (!rtpcp->rtp_exit_list.next)
+			INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
 	}
 
 	pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d.\n", rtp->name,
-- 
2.43.0

[PATCH AUTOSEL 6.7 05/11] rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 6b70399f9ef3809f6e308fd99dd78b072c1bd05c ]

This commit continues the elimination of deadlocks involving do_exit()
and RCU tasks by causing exit_tasks_rcu_start() to add the current
task to a per-CPU list and causing exit_tasks_rcu_stop() to remove the
current task from whatever list it is on.  These lists will be used to
track tasks that are exiting, while still accounting for any RCU-tasks
quiescent states that these tasks pass though.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@huawei.com/

Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tasks.h | 43 +++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index ac8eb667a6707..6d84cdbd2a0ec 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1151,25 +1151,48 @@ struct task_struct *get_rcu_tasks_gp_kthread(void)
 EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
 
 /*
- * Contribute to protect against tasklist scan blind spot while the
- * task is exiting and may be removed from the tasklist. See
- * corresponding synchronize_srcu() for further details.
+ * Protect against tasklist scan blind spot while the task is exiting and
+ * may be removed from the tasklist.  Do this by adding the task to yet
+ * another list.
+ *
+ * Note that the task will remove itself from this list, so there is no
+ * need for get_task_struct(), except in the case where rcu_tasks_pertask()
+ * adds it to the holdout list, in which case rcu_tasks_pertask() supplies
+ * the needed get_task_struct().
  */
-void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_start(void)
 {
-	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+	struct task_struct *t = current;
+
+	WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list));
+	preempt_disable();
+	rtpcp = this_cpu_ptr(rcu_tasks.rtpcpu);
+	t->rcu_tasks_exit_cpu = smp_processor_id();
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	if (!rtpcp->rtp_exit_list.next)
+		INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
+	list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	preempt_enable();
 }
 
 /*
- * Contribute to protect against tasklist scan blind spot while the
- * task is exiting and may be removed from the tasklist. See
- * corresponding synchronize_srcu() for further details.
+ * Remove the task from the "yet another list" because do_exit() is now
+ * non-preemptible, allowing synchronize_rcu() to wait beyond this point.
  */
-void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_stop(void)
 {
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
 	struct task_struct *t = current;
 
-	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
+	WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
+	rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	list_del_init(&t->rcu_tasks_exit_list);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 }
 
 /*
-- 
2.43.0

[PATCH AUTOSEL 6.7 06/11] rcu-tasks: Eliminate deadlocks involving do_exit() and RCU tasks

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 1612160b91272f5b1596f499584d6064bf5be794 ]

Holding a mutex across synchronize_rcu_tasks() and acquiring
that same mutex in code called from do_exit() after its call to
exit_tasks_rcu_start() but before its call to exit_tasks_rcu_stop()
results in deadlock.  This is by design, because tasks that are far
enough into do_exit() are no longer present on the tasks list, making
it a bit difficult for RCU Tasks to find them, let alone wait on them
to do a voluntary context switch.  However, such deadlocks are becoming
more frequent.  In addition, lockdep currently does not detect such
deadlocks and they can be difficult to reproduce.

In addition, if a task voluntarily context switches during that time
(for example, if it blocks acquiring a mutex), then this task is in an
RCU Tasks quiescent state.  And with some adjustments, RCU Tasks could
just as well take advantage of that fact.

This commit therefore eliminates these deadlock by replacing the
SRCU-based wait for do_exit() completion with per-CPU lists of tasks
currently exiting.  A given task will be on one of these per-CPU lists for
the same period of time that this task would previously have been in the
previous SRCU read-side critical section.  These lists enable RCU Tasks
to find the tasks that have already been removed from the tasks list,
but that must nevertheless be waited upon.

The RCU Tasks grace period gathers any of these do_exit() tasks that it
must wait on, and adds them to the list of holdouts.  Per-CPU locking
and get_task_struct() are used to synchronize addition to and removal
from these lists.

Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@huawei.com/

Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tasks.h | 44 ++++++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 6d84cdbd2a0ec..774408899e715 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -146,8 +146,6 @@ static struct rcu_tasks rt_name =							\
 }
 
 #ifdef CONFIG_TASKS_RCU
-/* Track exiting tasks in order to allow them to be waited for. */
-DEFINE_STATIC_SRCU(tasks_rcu_exit_srcu);
 
 /* Report delay in synchronize_srcu() completion in rcu_tasks_postscan(). */
 static void tasks_rcu_exit_srcu_stall(struct timer_list *unused);
@@ -855,10 +853,12 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 //	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.
+//	Gather per-CPU lists of tasks in do_exit() 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.  The synchronize_rcu() in rcu_tasks_postgp()
+//	will take care of any tasks stuck in the non-preemptible region
+//	of do_exit() following its call to exit_tasks_rcu_stop().
 // 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
@@ -871,8 +871,10 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 //	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().
+// exit_tasks_rcu_finish() functions add and remove, respectively, the
+// current task to a per-CPU list of tasks that rcu_tasks_postscan() must
+// wait on.  This is necessary because rcu_tasks_postscan() must wait on
+// tasks that have already been removed from the global list of tasks.
 //
 // Pre-grace-period update-side code is ordered before the grace
 // via the raw_spin_lock.*rcu_node().  Pre-grace-period read-side code
@@ -936,9 +938,13 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
 	}
 }
 
+void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
+DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
+
 /* Processing between scanning taskslist and draining the holdout list. */
 static void rcu_tasks_postscan(struct list_head *hop)
 {
+	int cpu;
 	int rtsi = READ_ONCE(rcu_task_stall_info);
 
 	if (!IS_ENABLED(CONFIG_TINY_RCU)) {
@@ -952,9 +958,9 @@ static void rcu_tasks_postscan(struct list_head *hop)
 	 * this, divide the fragile exit path part in two intersecting
 	 * read side critical sections:
 	 *
-	 * 1) An _SRCU_ read side starting before calling exit_notify(),
-	 *    which may remove the task from the tasklist, and ending after
-	 *    the final preempt_disable() call in do_exit().
+	 * 1) A task_struct list addition before calling exit_notify(),
+	 *    which may remove the task from the tasklist, with the
+	 *    removal after the final preempt_disable() call in do_exit().
 	 *
 	 * 2) An _RCU_ read side starting with the final preempt_disable()
 	 *    call in do_exit() and ending with the final call to schedule()
@@ -963,7 +969,17 @@ static void rcu_tasks_postscan(struct list_head *hop)
 	 * This handles the part 1). And postgp will handle part 2) with a
 	 * call to synchronize_rcu().
 	 */
-	synchronize_srcu(&tasks_rcu_exit_srcu);
+
+	for_each_possible_cpu(cpu) {
+		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu);
+		struct task_struct *t;
+
+		raw_spin_lock_irq_rcu_node(rtpcp);
+		list_for_each_entry(t, &rtpcp->rtp_exit_list, rcu_tasks_exit_list)
+			if (list_empty(&t->rcu_tasks_holdout_list))
+				rcu_tasks_pertask(t, hop);
+		raw_spin_unlock_irq_rcu_node(rtpcp);
+	}
 
 	if (!IS_ENABLED(CONFIG_TINY_RCU))
 		del_timer_sync(&tasks_rcu_exit_srcu_stall_timer);
@@ -1031,7 +1047,6 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp)
 	 *
 	 * In addition, this synchronize_rcu() waits for exiting tasks
 	 * to complete their final preempt_disable() region of execution,
-	 * cleaning up after synchronize_srcu(&tasks_rcu_exit_srcu),
 	 * enforcing the whole region before tasklist removal until
 	 * the final schedule() with TASK_DEAD state to be an RCU TASKS
 	 * read side critical section.
@@ -1039,9 +1054,6 @@ static void rcu_tasks_postgp(struct rcu_tasks *rtp)
 	synchronize_rcu();
 }
 
-void call_rcu_tasks(struct rcu_head *rhp, rcu_callback_t func);
-DEFINE_RCU_TASKS(rcu_tasks, rcu_tasks_wait_gp, call_rcu_tasks, "RCU Tasks");
-
 static void tasks_rcu_exit_srcu_stall(struct timer_list *unused)
 {
 #ifndef CONFIG_TINY_RCU
-- 
2.43.0

[PATCH AUTOSEL 6.7 07/11] rcu-tasks: Maintain real-time response in rcu_tasks_postscan()

[Sasha Levin]

From: "Paul E. McKenney" <paulmck@kernel.org>

[ Upstream commit 0bb11a372fc8d7006b4d0f42a2882939747bdbff ]

The current code will scan the entirety of each per-CPU list of exiting
tasks in ->rtp_exit_list with interrupts disabled.  This is normally just
fine, because each CPU typically won't have very many tasks in this state.
However, if a large number of tasks block late in do_exit(), these lists
could be arbitrarily long.  Low probability, perhaps, but it really
could happen.

This commit therefore occasionally re-enables interrupts while traversing
these lists, inserting a dummy element to hold the current place in the
list.  In kernels built with CONFIG_PREEMPT_RT=y, this re-enabling happens
after each list element is processed, otherwise every one-to-two jiffies.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Link: https://lore.kernel.org/all/ZdeI_-RfdLR8jlsm@localhost.localdomain/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tasks.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 774408899e715..4af68107544d3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -971,13 +971,33 @@ static void rcu_tasks_postscan(struct list_head *hop)
 	 */
 
 	for_each_possible_cpu(cpu) {
+		unsigned long j = jiffies + 1;
 		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu);
 		struct task_struct *t;
+		struct task_struct *t1;
+		struct list_head tmp;
 
 		raw_spin_lock_irq_rcu_node(rtpcp);
-		list_for_each_entry(t, &rtpcp->rtp_exit_list, rcu_tasks_exit_list)
+		list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) {
 			if (list_empty(&t->rcu_tasks_holdout_list))
 				rcu_tasks_pertask(t, hop);
+
+			// RT kernels need frequent pauses, otherwise
+			// pause at least once per pair of jiffies.
+			if (!IS_ENABLED(CONFIG_PREEMPT_RT) && time_before(jiffies, j))
+				continue;
+
+			// Keep our place in the list while pausing.
+			// Nothing else traverses this list, so adding a
+			// bare list_head is OK.
+			list_add(&tmp, &t->rcu_tasks_exit_list);
+			raw_spin_unlock_irq_rcu_node(rtpcp);
+			cond_resched(); // For CONFIG_PREEMPT=n kernels
+			raw_spin_lock_irq_rcu_node(rtpcp);
+			t1 = list_entry(tmp.next, struct task_struct, rcu_tasks_exit_list);
+			list_del(&tmp);
+			j = jiffies + 1;
+		}
 		raw_spin_unlock_irq_rcu_node(rtpcp);
 	}
 
-- 
2.43.0

[PATCH AUTOSEL 6.7 10/11] rcu/nocb: Fix WARN_ON_ONCE() in the rcu_nocb_bypass_lock()

[Sasha Levin]

From: Zqiang <qiang.zhang1211@gmail.com>

[ Upstream commit dda98810b552fc6bf650f4270edeebdc2f28bd3f ]

For the kernels built with CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y and
CONFIG_RCU_LAZY=y, the following scenarios will trigger WARN_ON_ONCE()
in the rcu_nocb_bypass_lock() and rcu_nocb_wait_contended() functions:

        CPU2                                               CPU11
kthread
rcu_nocb_cb_kthread                                       ksys_write
rcu_do_batch                                              vfs_write
rcu_torture_timer_cb                                      proc_sys_write
__kmem_cache_free                                         proc_sys_call_handler
kmemleak_free                                             drop_caches_sysctl_handler
delete_object_full                                        drop_slab
__delete_object                                           shrink_slab
put_object                                                lazy_rcu_shrink_scan
call_rcu                                                  rcu_nocb_flush_bypass
__call_rcu_commn                                            rcu_nocb_bypass_lock
                                                            raw_spin_trylock(&rdp->nocb_bypass_lock) fail
                                                            atomic_inc(&rdp->nocb_lock_contended);
rcu_nocb_wait_contended                                     WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
 WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended))                                          |
                            |_ _ _ _ _ _ _ _ _ _same rdp and rdp->cpu != 11_ _ _ _ _ _ _ _ _ __|

Reproduce this bug with "echo 3 > /proc/sys/vm/drop_caches".

This commit therefore uses rcu_nocb_try_flush_bypass() instead of
rcu_nocb_flush_bypass() in lazy_rcu_shrink_scan().  If the nocb_bypass
queue is being flushed, then rcu_nocb_try_flush_bypass will return
directly.

Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/rcu/tree_nocb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 4efbf7333d4e1..d430b4656f59e 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1383,7 +1383,7 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
 			rcu_nocb_unlock_irqrestore(rdp, flags);
 			continue;
 		}
-		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
+		rcu_nocb_try_flush_bypass(rdp, jiffies);
 		rcu_nocb_unlock_irqrestore(rdp, flags);
 		wake_nocb_gp(rdp, false);
 		sc->nr_to_scan -= _count;
-- 
2.43.0