[PATCH v6 0/3] Introduce put_task_struct_atomic_sleep()

[PATCH v6 0/3] Introduce put_task_struct_atomic_sleep()

[Wander Lairson Costa]

The put_task_struct() function reduces a usage counter and invokes
__put_task_struct() when the counter reaches zero.

In the case of __put_task_struct(), it indirectly acquires a spinlock,
which operates as a sleeping lock under the PREEMPT_RT configuration.
As a result, invoking put_task_struct() within an atomic context is
not feasible for real-time (RT) kernels.

To address this issue, this patch series introduces a new function
called put_task_struct_atomic_safe(). When compiled with the
PREEMPT_RT configuration, this function defers the call to
__put_task_struct() to a process context.

Additionally, the patch series rectifies known problematic call sites
to ensure smooth functioning.

Changelog
=========

v1:
* Initial implementation fixing the splat.

v2:
* Isolate the logic in its own function.
* Fix two more cases caught in review.

v3:
* Change __put_task_struct() to handle the issue internally.

v4:
* Explain why call_rcu() is safe to call from interrupt context.

v5:
* Explain why __put_task_struct() doesn't conflict with
  put_task_sruct_rcu_user.

v6:
* As per Sebastian's review, revert back the implementation of v2
  with a distinct function.
* Add a check in put_task_struct() to warning when called from a
  non-sleepable context.
* Address more call sites.

Wander Lairson Costa (3):
  sched/core: warn on call put_task_struct in invalid context
  sched/task: Add the put_task_struct_atomic_safe() function
  treewide: replace put_task_struct() witht the atomic safe version

 include/linux/sched/task.h | 45 ++++++++++++++++++++++++++++++++++++++
 kernel/events/core.c       |  6 ++---
 kernel/fork.c              |  8 +++++++
 kernel/locking/rtmutex.c   | 10 ++++-----
 kernel/sched/core.c        |  6 ++---
 kernel/sched/deadline.c    | 16 +++++++-------
 kernel/sched/rt.c          |  4 ++--
 7 files changed, 74 insertions(+), 21 deletions(-)

-- 
2.39.2

[PATCH v6 1/3] sched/core: warn on call put_task_struct in invalid context

[Wander Lairson Costa]

Under PREEMPT_RT, spinlocks become sleepable locks. put_task_struct()
indirectly acquires a spinlock. Therefore, it can't be called in
atomic/interrupt context in RT kernels.

To prevent such conditions, add a check for atomic/interrupt context
before calling put_task_struct().

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/sched/task.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 357e0068497c..b597b97b1f8f 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -113,14 +113,28 @@ static inline struct task_struct *get_task_struct(struct task_struct *t)
 
 extern void __put_task_struct(struct task_struct *t);
 
+#define PUT_TASK_RESCHED_OFFSETS \
+	(rcu_preempt_depth() << MIGHT_RESCHED_RCU_SHIFT)
+
+#define __put_task_might_resched() \
+	__might_resched(__FILE__, __LINE__, PUT_TASK_RESCHED_OFFSETS)
+
+#define put_task_might_resched()			\
+	do {						\
+		if (IS_ENABLED(CONFIG_PREEMPT_RT))	\
+			__put_task_might_resched();	\
+	} while (0)
+
 static inline void put_task_struct(struct task_struct *t)
 {
+	put_task_might_resched();
 	if (refcount_dec_and_test(&t->usage))
 		__put_task_struct(t);
 }
 
 static inline void put_task_struct_many(struct task_struct *t, int nr)
 {
+	put_task_might_resched();
 	if (refcount_sub_and_test(nr, &t->usage))
 		__put_task_struct(t);
 }
-- 
2.39.2

[PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

Due to the possibility of indirectly acquiring sleeping locks, it is
unsafe to call put_task_struct() in atomic contexts when the kernel is
compiled with PREEMPT_RT.

To mitigate this issue, this commit introduces
put_task_struct_atomic_safe(), which schedules __put_task_struct()
through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
be a more natural approach, we cannot allocate dynamic memory from
atomic context in PREEMPT_RT, making the code more complex.

This implementation ensures safe execution in atomic contexts and
avoids any potential issues that may arise from using the non-atomic
version.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reported-by: Hu Chunyu <chuhu@redhat.com>
Cc: Paul McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
 kernel/fork.c              |  8 ++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index b597b97b1f8f..5c13b83d7008 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
 
 void put_task_struct_rcu_user(struct task_struct *task);
 
+extern void __delayed_put_task_struct(struct rcu_head *rhp);
+
+static inline void put_task_struct_atomic_safe(struct task_struct *task)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		/*
+		 * Decrement the refcount explicitly to avoid unnecessarily
+		 * calling call_rcu.
+		 */
+		if (refcount_dec_and_test(&task->usage))
+			/*
+			 * under PREEMPT_RT, we can't call put_task_struct
+			 * in atomic context because it will indirectly
+			 * acquire sleeping locks.
+			 * call_rcu() will schedule delayed_put_task_struct_rcu()
+			 * to be called in process context.
+			 *
+			 * __put_task_struct() is called called when
+			 * refcount_dec_and_test(&t->usage) succeeds.
+			 *
+			 * This means that it can't "conflict" with
+			 * put_task_struct_rcu_user() which abuses ->rcu the same
+			 * way; rcu_users has a reference so task->usage can't be
+			 * zero after rcu_users 1 -> 0 transition.
+			 */
+			call_rcu(&task->rcu, __delayed_put_task_struct);
+	} else {
+		put_task_struct(task);
+	}
+}
+
 /* Free all architecture-specific resources held by a thread. */
 void release_thread(struct task_struct *dead_task);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 0c92f224c68c..9884794fe4b8 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
 }
 EXPORT_SYMBOL_GPL(__put_task_struct);
 
+void __delayed_put_task_struct(struct rcu_head *rhp)
+{
+	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+	__put_task_struct(task);
+}
+EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
+
 void __init __weak arch_task_cache_init(void) { }
 
 /*
-- 
2.39.2

[PATCH v6 3/3] treewide: replace put_task_struct() witht the atomic safe version

[Wander Lairson Costa]

In places where put_task_struct() is called in a non-sleepable context,
we replace those calls by put_task_struct_atomic_safe().

These call sites were found by running internal regression tests and
looking for warnings generated by put_task_might_resched().

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Cc: Valentin Schneider <vschneid@redhat.com>
---
 kernel/events/core.c     |  6 +++---
 kernel/locking/rtmutex.c | 10 +++++-----
 kernel/sched/core.c      |  6 +++---
 kernel/sched/deadline.c  | 16 ++++++++--------
 kernel/sched/rt.c        |  4 ++--
 5 files changed, 21 insertions(+), 21 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 435815d3be3f..8f823da02324 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1181,7 +1181,7 @@ static void put_ctx(struct perf_event_context *ctx)
 		if (ctx->parent_ctx)
 			put_ctx(ctx->parent_ctx);
 		if (ctx->task && ctx->task != TASK_TOMBSTONE)
-			put_task_struct(ctx->task);
+			put_task_struct_atomic_safe(ctx->task);
 		call_rcu(&ctx->rcu_head, free_ctx);
 	}
 }
@@ -13019,7 +13019,7 @@ static void perf_event_exit_task_context(struct task_struct *child)
 	RCU_INIT_POINTER(child->perf_event_ctxp, NULL);
 	put_ctx(child_ctx); /* cannot be last */
 	WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE);
-	put_task_struct(current); /* cannot be last */
+	put_task_struct_atomic_safe(current); /* cannot be last */
 
 	clone_ctx = unclone_ctx(child_ctx);
 	raw_spin_unlock_irq(&child_ctx->lock);
@@ -13124,7 +13124,7 @@ void perf_event_free_task(struct task_struct *task)
 	 */
 	RCU_INIT_POINTER(task->perf_event_ctxp, NULL);
 	WRITE_ONCE(ctx->task, TASK_TOMBSTONE);
-	put_task_struct(task); /* cannot be last */
+	put_task_struct_atomic_safe(task); /* cannot be last */
 	raw_spin_unlock_irq(&ctx->lock);
 
 
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 728f434de2bb..3ecb8d6ae039 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -509,7 +509,7 @@ static __always_inline void rt_mutex_wake_up_q(struct rt_wake_q_head *wqh)
 {
 	if (IS_ENABLED(CONFIG_PREEMPT_RT) && wqh->rtlock_task) {
 		wake_up_state(wqh->rtlock_task, TASK_RTLOCK_WAIT);
-		put_task_struct(wqh->rtlock_task);
+		put_task_struct_atomic_safe(wqh->rtlock_task);
 		wqh->rtlock_task = NULL;
 	}
 
@@ -649,7 +649,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 			       "task: %s (%d)\n", max_lock_depth,
 			       top_task->comm, task_pid_nr(top_task));
 		}
-		put_task_struct(task);
+		put_task_struct_atomic_safe(task);
 
 		return -EDEADLK;
 	}
@@ -817,7 +817,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 		 * No requeue[7] here. Just release @task [8]
 		 */
 		raw_spin_unlock(&task->pi_lock);
-		put_task_struct(task);
+		put_task_struct_atomic_safe(task);
 
 		/*
 		 * [9] check_exit_conditions_3 protected by lock->wait_lock.
@@ -886,7 +886,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
 
 	/* [8] Release the task */
 	raw_spin_unlock(&task->pi_lock);
-	put_task_struct(task);
+	put_task_struct_atomic_safe(task);
 
 	/*
 	 * [9] check_exit_conditions_3 protected by lock->wait_lock.
@@ -990,7 +990,7 @@ static int __sched rt_mutex_adjust_prio_chain(struct task_struct *task,
  out_unlock_pi:
 	raw_spin_unlock_irq(&task->pi_lock);
  out_put_task:
-	put_task_struct(task);
+	put_task_struct_atomic_safe(task);
 
 	return ret;
 }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d18c3969f90..a4783f0c9f01 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1007,7 +1007,7 @@ void wake_up_q(struct wake_q_head *head)
 		 * the queueing in wake_q_add() so as not to miss wakeups.
 		 */
 		wake_up_process(task);
-		put_task_struct(task);
+		put_task_struct_atomic_safe(task);
 	}
 }
 
@@ -2528,7 +2528,7 @@ int push_cpu_stop(void *arg)
 	raw_spin_rq_unlock(rq);
 	raw_spin_unlock_irq(&p->pi_lock);
 
-	put_task_struct(p);
+	put_task_struct_atomic_safe(p);
 	return 0;
 }
 
@@ -9316,7 +9316,7 @@ static int __balance_push_cpu_stop(void *arg)
 	rq_unlock(rq, &rf);
 	raw_spin_unlock_irq(&p->pi_lock);
 
-	put_task_struct(p);
+	put_task_struct_atomic_safe(p);
 
 	return 0;
 }
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 71b24371a6f7..0f8b8a490dc0 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -327,7 +327,7 @@ static void dl_change_utilization(struct task_struct *p, u64 new_bw)
 		 * so we are still safe.
 		 */
 		if (hrtimer_try_to_cancel(&p->dl.inactive_timer) == 1)
-			put_task_struct(p);
+			put_task_struct_atomic_safe(p);
 	}
 	__sub_rq_bw(p->dl.dl_bw, &rq->dl);
 	__add_rq_bw(new_bw, &rq->dl);
@@ -467,7 +467,7 @@ static void task_contending(struct sched_dl_entity *dl_se, int flags)
 		 * so we are still safe.
 		 */
 		if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == 1)
-			put_task_struct(dl_task_of(dl_se));
+			put_task_struct_atomic_safe(dl_task_of(dl_se));
 	} else {
 		/*
 		 * Since "dl_non_contending" is not set, the
@@ -1207,7 +1207,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
 	 * This can free the task_struct, including this hrtimer, do not touch
 	 * anything related to that after this.
 	 */
-	put_task_struct(p);
+	put_task_struct_atomic_safe(p);
 
 	return HRTIMER_NORESTART;
 }
@@ -1442,7 +1442,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 	dl_se->dl_non_contending = 0;
 unlock:
 	task_rq_unlock(rq, p, &rf);
-	put_task_struct(p);
+	put_task_struct_atomic_safe(p);
 
 	return HRTIMER_NORESTART;
 }
@@ -1899,7 +1899,7 @@ static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused
 		 * so we are still safe.
 		 */
 		if (hrtimer_try_to_cancel(&p->dl.inactive_timer) == 1)
-			put_task_struct(p);
+			put_task_struct_atomic_safe(p);
 	}
 	sub_rq_bw(&p->dl, &rq->dl);
 	rq_unlock(rq, &rf);
@@ -2351,7 +2351,7 @@ static int push_dl_task(struct rq *rq)
 			/* No more tasks */
 			goto out;
 
-		put_task_struct(next_task);
+		put_task_struct_atomic_safe(next_task);
 		next_task = task;
 		goto retry;
 	}
@@ -2366,7 +2366,7 @@ static int push_dl_task(struct rq *rq)
 	double_unlock_balance(rq, later_rq);
 
 out:
-	put_task_struct(next_task);
+	put_task_struct_atomic_safe(next_task);
 
 	return ret;
 }
@@ -2633,7 +2633,7 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
 static void switched_to_dl(struct rq *rq, struct task_struct *p)
 {
 	if (hrtimer_try_to_cancel(&p->dl.inactive_timer) == 1)
-		put_task_struct(p);
+		put_task_struct_atomic_safe(p);
 
 	/* If p is not queued we will update its parameters at next wakeup. */
 	if (!task_on_rq_queued(p)) {
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 0a11f44adee5..e58a84535f61 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2150,7 +2150,7 @@ static int push_rt_task(struct rq *rq, bool pull)
 		/*
 		 * Something has shifted, try again.
 		 */
-		put_task_struct(next_task);
+		put_task_struct_atomic_safe(next_task);
 		next_task = task;
 		goto retry;
 	}
@@ -2163,7 +2163,7 @@ static int push_rt_task(struct rq *rq, bool pull)
 
 	double_unlock_balance(rq, lowest_rq);
 out:
-	put_task_struct(next_task);
+	put_task_struct_atomic_safe(next_task);
 
 	return ret;
 }
-- 
2.39.2

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Waiman Long]

On 4/14/23 08:55, Wander Lairson Costa wrote:
> Due to the possibility of indirectly acquiring sleeping locks, it is
> unsafe to call put_task_struct() in atomic contexts when the kernel is
> compiled with PREEMPT_RT.
>
> To mitigate this issue, this commit introduces
> put_task_struct_atomic_safe(), which schedules __put_task_struct()
> through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> be a more natural approach, we cannot allocate dynamic memory from
> atomic context in PREEMPT_RT, making the code more complex.
>
> This implementation ensures safe execution in atomic contexts and
> avoids any potential issues that may arise from using the non-atomic
> version.
>
> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> Reported-by: Hu Chunyu <chuhu@redhat.com>
> Cc: Paul McKenney <paulmck@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>   include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
>   kernel/fork.c              |  8 ++++++++
>   2 files changed, 39 insertions(+)
>
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index b597b97b1f8f..5c13b83d7008 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
>   
>   void put_task_struct_rcu_user(struct task_struct *task);
>   
> +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> +
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/*
> +		 * Decrement the refcount explicitly to avoid unnecessarily
> +		 * calling call_rcu.
> +		 */
> +		if (refcount_dec_and_test(&task->usage))
> +			/*
> +			 * under PREEMPT_RT, we can't call put_task_struct
> +			 * in atomic context because it will indirectly
> +			 * acquire sleeping locks.
> +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
delayed_put_task_struct_rcu()?
> +			 * to be called in process context.
> +			 *
> +			 * __put_task_struct() is called called when
"called called"?
> +			 * refcount_dec_and_test(&t->usage) succeeds.
> +			 *
> +			 * This means that it can't "conflict" with
> +			 * put_task_struct_rcu_user() which abuses ->rcu the same
> +			 * way; rcu_users has a reference so task->usage can't be
> +			 * zero after rcu_users 1 -> 0 transition.

Note that put_task_struct_rcu_user() isn't the only user of task->rcu. 
delayed_free_task() in kernel/fork.c also uses it, though it is only 
called in the error case. Still you may need to take a look to make sure 
that there is no conflict.

Cheers,
Longman

Re: [PATCH v6 3/3] treewide: replace put_task_struct() witht the atomic safe version

[Waiman Long]

On 4/14/23 08:55, Wander Lairson Costa wrote:
> In places where put_task_struct() is called in a non-sleepable context,
> we replace those calls by put_task_struct_atomic_safe().
>
> These call sites were found by running internal regression tests and
> looking for warnings generated by put_task_might_resched().
>
> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> ---
>   kernel/events/core.c     |  6 +++---
>   kernel/locking/rtmutex.c | 10 +++++-----
>   kernel/sched/core.c      |  6 +++---
>   kernel/sched/deadline.c  | 16 ++++++++--------
>   kernel/sched/rt.c        |  4 ++--
>   5 files changed, 21 insertions(+), 21 deletions(-)

Typo "witht" in your patch title.

Cheers,
Longman

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

On Mon, Apr 17, 2023 at 02:51:45PM -0400, Waiman Long wrote:
> 
> On 4/14/23 08:55, Wander Lairson Costa wrote:
> > Due to the possibility of indirectly acquiring sleeping locks, it is
> > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > compiled with PREEMPT_RT.
> > 
> > To mitigate this issue, this commit introduces
> > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > be a more natural approach, we cannot allocate dynamic memory from
> > atomic context in PREEMPT_RT, making the code more complex.
> > 
> > This implementation ensures safe execution in atomic contexts and
> > avoids any potential issues that may arise from using the non-atomic
> > version.
> > 
> > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > Cc: Paul McKenney <paulmck@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >   include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> >   kernel/fork.c              |  8 ++++++++
> >   2 files changed, 39 insertions(+)
> > 
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index b597b97b1f8f..5c13b83d7008 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> >   void put_task_struct_rcu_user(struct task_struct *task);
> > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > +
> > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > +{
> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		/*
> > +		 * Decrement the refcount explicitly to avoid unnecessarily
> > +		 * calling call_rcu.
> > +		 */
> > +		if (refcount_dec_and_test(&task->usage))
> > +			/*
> > +			 * under PREEMPT_RT, we can't call put_task_struct
> > +			 * in atomic context because it will indirectly
> > +			 * acquire sleeping locks.
> > +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
> delayed_put_task_struct_rcu()?
> > +			 * to be called in process context.
> > +			 *
> > +			 * __put_task_struct() is called called when
> "called called"?
> > +			 * refcount_dec_and_test(&t->usage) succeeds.
> > +			 *
> > +			 * This means that it can't "conflict" with
> > +			 * put_task_struct_rcu_user() which abuses ->rcu the same
> > +			 * way; rcu_users has a reference so task->usage can't be
> > +			 * zero after rcu_users 1 -> 0 transition.
> 
> Note that put_task_struct_rcu_user() isn't the only user of task->rcu.
> delayed_free_task() in kernel/fork.c also uses it, though it is only called
> in the error case. Still you may need to take a look to make sure that there
> is no conflict.
> 

delayed_free_task() is called when a process fails to start. Therefore, AFAICT,
there is no way it can conflict with put_task_struct().

> Cheers,
> Longman
> 

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Waiman Long]

On 4/18/23 10:18, Wander Lairson Costa wrote:
> On Mon, Apr 17, 2023 at 02:51:45PM -0400, Waiman Long wrote:
>> On 4/14/23 08:55, Wander Lairson Costa wrote:
>>> Due to the possibility of indirectly acquiring sleeping locks, it is
>>> unsafe to call put_task_struct() in atomic contexts when the kernel is
>>> compiled with PREEMPT_RT.
>>>
>>> To mitigate this issue, this commit introduces
>>> put_task_struct_atomic_safe(), which schedules __put_task_struct()
>>> through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
>>> be a more natural approach, we cannot allocate dynamic memory from
>>> atomic context in PREEMPT_RT, making the code more complex.
>>>
>>> This implementation ensures safe execution in atomic contexts and
>>> avoids any potential issues that may arise from using the non-atomic
>>> version.
>>>
>>> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
>>> Reported-by: Hu Chunyu <chuhu@redhat.com>
>>> Cc: Paul McKenney <paulmck@kernel.org>
>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>> ---
>>>    include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
>>>    kernel/fork.c              |  8 ++++++++
>>>    2 files changed, 39 insertions(+)
>>>
>>> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
>>> index b597b97b1f8f..5c13b83d7008 100644
>>> --- a/include/linux/sched/task.h
>>> +++ b/include/linux/sched/task.h
>>> @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
>>>    void put_task_struct_rcu_user(struct task_struct *task);
>>> +extern void __delayed_put_task_struct(struct rcu_head *rhp);
>>> +
>>> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
>>> +{
>>> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
>>> +		/*
>>> +		 * Decrement the refcount explicitly to avoid unnecessarily
>>> +		 * calling call_rcu.
>>> +		 */
>>> +		if (refcount_dec_and_test(&task->usage))
>>> +			/*
>>> +			 * under PREEMPT_RT, we can't call put_task_struct
>>> +			 * in atomic context because it will indirectly
>>> +			 * acquire sleeping locks.
>>> +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
>> delayed_put_task_struct_rcu()?
>>> +			 * to be called in process context.
>>> +			 *
>>> +			 * __put_task_struct() is called called when
>> "called called"?
>>> +			 * refcount_dec_and_test(&t->usage) succeeds.
>>> +			 *
>>> +			 * This means that it can't "conflict" with
>>> +			 * put_task_struct_rcu_user() which abuses ->rcu the same
>>> +			 * way; rcu_users has a reference so task->usage can't be
>>> +			 * zero after rcu_users 1 -> 0 transition.
>> Note that put_task_struct_rcu_user() isn't the only user of task->rcu.
>> delayed_free_task() in kernel/fork.c also uses it, though it is only called
>> in the error case. Still you may need to take a look to make sure that there
>> is no conflict.
>>
> delayed_free_task() is called when a process fails to start. Therefore, AFAICT,
> there is no way it can conflict with put_task_struct().

I think so too, but for completeness, you should document somewhere that 
it is a possible conflicting user.

Cheers,
Longman

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Paul E. McKenney]

On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> Due to the possibility of indirectly acquiring sleeping locks, it is
> unsafe to call put_task_struct() in atomic contexts when the kernel is
> compiled with PREEMPT_RT.
> 
> To mitigate this issue, this commit introduces
> put_task_struct_atomic_safe(), which schedules __put_task_struct()
> through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> be a more natural approach, we cannot allocate dynamic memory from
> atomic context in PREEMPT_RT, making the code more complex.
> 
> This implementation ensures safe execution in atomic contexts and
> avoids any potential issues that may arise from using the non-atomic
> version.
> 
> Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> Reported-by: Hu Chunyu <chuhu@redhat.com>
> Cc: Paul McKenney <paulmck@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> ---
>  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
>  kernel/fork.c              |  8 ++++++++
>  2 files changed, 39 insertions(+)
> 
> diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> index b597b97b1f8f..5c13b83d7008 100644
> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
>  
>  void put_task_struct_rcu_user(struct task_struct *task);
>  
> +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> +
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/*
> +		 * Decrement the refcount explicitly to avoid unnecessarily
> +		 * calling call_rcu.
> +		 */
> +		if (refcount_dec_and_test(&task->usage))
> +			/*
> +			 * under PREEMPT_RT, we can't call put_task_struct
> +			 * in atomic context because it will indirectly
> +			 * acquire sleeping locks.
> +			 * call_rcu() will schedule delayed_put_task_struct_rcu()
> +			 * to be called in process context.
> +			 *
> +			 * __put_task_struct() is called called when
> +			 * refcount_dec_and_test(&t->usage) succeeds.
> +			 *
> +			 * This means that it can't "conflict" with
> +			 * put_task_struct_rcu_user() which abuses ->rcu the same
> +			 * way; rcu_users has a reference so task->usage can't be
> +			 * zero after rcu_users 1 -> 0 transition.
> +			 */
> +			call_rcu(&task->rcu, __delayed_put_task_struct);

This will invoke __delayed_put_task_struct() with softirqs disabled.
Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?

							Thanx, Paul

> +	} else {
> +		put_task_struct(task);
> +	}
> +}
> +
>  /* Free all architecture-specific resources held by a thread. */
>  void release_thread(struct task_struct *dead_task);
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0c92f224c68c..9884794fe4b8 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
>  }
>  EXPORT_SYMBOL_GPL(__put_task_struct);
>  
> +void __delayed_put_task_struct(struct rcu_head *rhp)
> +{
> +	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> +
> +	__put_task_struct(task);
> +}
> +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> +
>  void __init __weak arch_task_cache_init(void) { }
>  
>  /*
> -- 
> 2.39.2
> 

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

On Mon, Apr 24, 2023 at 3:09 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> > Due to the possibility of indirectly acquiring sleeping locks, it is
> > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > compiled with PREEMPT_RT.
> >
> > To mitigate this issue, this commit introduces
> > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > be a more natural approach, we cannot allocate dynamic memory from
> > atomic context in PREEMPT_RT, making the code more complex.
> >
> > This implementation ensures safe execution in atomic contexts and
> > avoids any potential issues that may arise from using the non-atomic
> > version.
> >
> > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > Cc: Paul McKenney <paulmck@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > ---
> >  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> >  kernel/fork.c              |  8 ++++++++
> >  2 files changed, 39 insertions(+)
> >
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index b597b97b1f8f..5c13b83d7008 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> >
> >  void put_task_struct_rcu_user(struct task_struct *task);
> >
> > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > +
> > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > +{
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +             /*
> > +              * Decrement the refcount explicitly to avoid unnecessarily
> > +              * calling call_rcu.
> > +              */
> > +             if (refcount_dec_and_test(&task->usage))
> > +                     /*
> > +                      * under PREEMPT_RT, we can't call put_task_struct
> > +                      * in atomic context because it will indirectly
> > +                      * acquire sleeping locks.
> > +                      * call_rcu() will schedule delayed_put_task_struct_rcu()
> > +                      * to be called in process context.
> > +                      *
> > +                      * __put_task_struct() is called called when
> > +                      * refcount_dec_and_test(&t->usage) succeeds.
> > +                      *
> > +                      * This means that it can't "conflict" with
> > +                      * put_task_struct_rcu_user() which abuses ->rcu the same
> > +                      * way; rcu_users has a reference so task->usage can't be
> > +                      * zero after rcu_users 1 -> 0 transition.
> > +                      */
> > +                     call_rcu(&task->rcu, __delayed_put_task_struct);
>
> This will invoke __delayed_put_task_struct() with softirqs disabled.
> Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
>

softirqs execute in thread context in PREEMPT_RT. We are good here.

>                                                         Thanx, Paul
>
> > +     } else {
> > +             put_task_struct(task);
> > +     }
> > +}
> > +
> >  /* Free all architecture-specific resources held by a thread. */
> >  void release_thread(struct task_struct *dead_task);
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 0c92f224c68c..9884794fe4b8 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
> >  }
> >  EXPORT_SYMBOL_GPL(__put_task_struct);
> >
> > +void __delayed_put_task_struct(struct rcu_head *rhp)
> > +{
> > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > +     __put_task_struct(task);
> > +}
> > +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> > +
> >  void __init __weak arch_task_cache_init(void) { }
> >
> >  /*
> > --
> > 2.39.2
> >
>

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Paul E. McKenney]

On Mon, Apr 24, 2023 at 03:43:09PM -0300, Wander Lairson Costa wrote:
> On Mon, Apr 24, 2023 at 3:09 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> > > Due to the possibility of indirectly acquiring sleeping locks, it is
> > > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > > compiled with PREEMPT_RT.
> > >
> > > To mitigate this issue, this commit introduces
> > > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > > be a more natural approach, we cannot allocate dynamic memory from
> > > atomic context in PREEMPT_RT, making the code more complex.
> > >
> > > This implementation ensures safe execution in atomic contexts and
> > > avoids any potential issues that may arise from using the non-atomic
> > > version.
> > >
> > > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > > Cc: Paul McKenney <paulmck@kernel.org>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > ---
> > >  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> > >  kernel/fork.c              |  8 ++++++++
> > >  2 files changed, 39 insertions(+)
> > >
> > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > index b597b97b1f8f..5c13b83d7008 100644
> > > --- a/include/linux/sched/task.h
> > > +++ b/include/linux/sched/task.h
> > > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> > >
> > >  void put_task_struct_rcu_user(struct task_struct *task);
> > >
> > > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > > +
> > > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > > +{
> > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > > +             /*
> > > +              * Decrement the refcount explicitly to avoid unnecessarily
> > > +              * calling call_rcu.
> > > +              */
> > > +             if (refcount_dec_and_test(&task->usage))
> > > +                     /*
> > > +                      * under PREEMPT_RT, we can't call put_task_struct
> > > +                      * in atomic context because it will indirectly
> > > +                      * acquire sleeping locks.
> > > +                      * call_rcu() will schedule delayed_put_task_struct_rcu()
> > > +                      * to be called in process context.
> > > +                      *
> > > +                      * __put_task_struct() is called called when
> > > +                      * refcount_dec_and_test(&t->usage) succeeds.
> > > +                      *
> > > +                      * This means that it can't "conflict" with
> > > +                      * put_task_struct_rcu_user() which abuses ->rcu the same
> > > +                      * way; rcu_users has a reference so task->usage can't be
> > > +                      * zero after rcu_users 1 -> 0 transition.
> > > +                      */
> > > +                     call_rcu(&task->rcu, __delayed_put_task_struct);
> >
> > This will invoke __delayed_put_task_struct() with softirqs disabled.
> > Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
> 
> softirqs execute in thread context in PREEMPT_RT. We are good here.

So the sleeping lock is a spinlock rather than (say) a mutex?

                                                        Thanx, Paul

> > > +     } else {
> > > +             put_task_struct(task);
> > > +     }
> > > +}
> > > +
> > >  /* Free all architecture-specific resources held by a thread. */
> > >  void release_thread(struct task_struct *dead_task);
> > >
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 0c92f224c68c..9884794fe4b8 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
> > >  }
> > >  EXPORT_SYMBOL_GPL(__put_task_struct);
> > >
> > > +void __delayed_put_task_struct(struct rcu_head *rhp)
> > > +{
> > > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > > +
> > > +     __put_task_struct(task);
> > > +}
> > > +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> > > +
> > >  void __init __weak arch_task_cache_init(void) { }
> > >
> > >  /*
> > > --
> > > 2.39.2
> > >
> >
> 

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

On Mon, Apr 24, 2023 at 3:52 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Mon, Apr 24, 2023 at 03:43:09PM -0300, Wander Lairson Costa wrote:
> > On Mon, Apr 24, 2023 at 3:09 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > >
> > > On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> > > > Due to the possibility of indirectly acquiring sleeping locks, it is
> > > > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > > > compiled with PREEMPT_RT.
> > > >
> > > > To mitigate this issue, this commit introduces
> > > > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > > > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > > > be a more natural approach, we cannot allocate dynamic memory from
> > > > atomic context in PREEMPT_RT, making the code more complex.
> > > >
> > > > This implementation ensures safe execution in atomic contexts and
> > > > avoids any potential issues that may arise from using the non-atomic
> > > > version.
> > > >
> > > > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > > > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > > > Cc: Paul McKenney <paulmck@kernel.org>
> > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > ---
> > > >  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> > > >  kernel/fork.c              |  8 ++++++++
> > > >  2 files changed, 39 insertions(+)
> > > >
> > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > > index b597b97b1f8f..5c13b83d7008 100644
> > > > --- a/include/linux/sched/task.h
> > > > +++ b/include/linux/sched/task.h
> > > > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> > > >
> > > >  void put_task_struct_rcu_user(struct task_struct *task);
> > > >
> > > > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > > > +
> > > > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > > > +{
> > > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > > > +             /*
> > > > +              * Decrement the refcount explicitly to avoid unnecessarily
> > > > +              * calling call_rcu.
> > > > +              */
> > > > +             if (refcount_dec_and_test(&task->usage))
> > > > +                     /*
> > > > +                      * under PREEMPT_RT, we can't call put_task_struct
> > > > +                      * in atomic context because it will indirectly
> > > > +                      * acquire sleeping locks.
> > > > +                      * call_rcu() will schedule delayed_put_task_struct_rcu()
> > > > +                      * to be called in process context.
> > > > +                      *
> > > > +                      * __put_task_struct() is called called when
> > > > +                      * refcount_dec_and_test(&t->usage) succeeds.
> > > > +                      *
> > > > +                      * This means that it can't "conflict" with
> > > > +                      * put_task_struct_rcu_user() which abuses ->rcu the same
> > > > +                      * way; rcu_users has a reference so task->usage can't be
> > > > +                      * zero after rcu_users 1 -> 0 transition.
> > > > +                      */
> > > > +                     call_rcu(&task->rcu, __delayed_put_task_struct);
> > >
> > > This will invoke __delayed_put_task_struct() with softirqs disabled.
> > > Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
> >
> > softirqs execute in thread context in PREEMPT_RT. We are good here.
>
> So the sleeping lock is a spinlock rather than (say) a mutex?
>

Yes, under PREEMPT_RT, spinlocks are implemented in terms of rtmutex.

>                                                         Thanx, Paul
>
> > > > +     } else {
> > > > +             put_task_struct(task);
> > > > +     }
> > > > +}
> > > > +
> > > >  /* Free all architecture-specific resources held by a thread. */
> > > >  void release_thread(struct task_struct *dead_task);
> > > >
> > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > index 0c92f224c68c..9884794fe4b8 100644
> > > > --- a/kernel/fork.c
> > > > +++ b/kernel/fork.c
> > > > @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(__put_task_struct);
> > > >
> > > > +void __delayed_put_task_struct(struct rcu_head *rhp)
> > > > +{
> > > > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > > > +
> > > > +     __put_task_struct(task);
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> > > > +
> > > >  void __init __weak arch_task_cache_init(void) { }
> > > >
> > > >  /*
> > > > --
> > > > 2.39.2
> > > >
> > >
> >
>

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Steven Rostedt]

On Mon, 24 Apr 2023 11:52:52 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> > softirqs execute in thread context in PREEMPT_RT. We are good here.  
> 
> So the sleeping lock is a spinlock rather than (say) a mutex?

local_bh_disable() on RT basically turns into:

	local_lock(&softirq_ctrl.lock);
	rcu_read_lock();

Which grabs a per CPU mutex that is taken by softirqs, and also calls
rcu_read_lock(). This allows bottom halves to still run as threads but
maintain the same synchronization as they do on mainline.

-- Steve

Re: [PATCH v6 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Paul E. McKenney]

On Mon, Apr 24, 2023 at 05:34:29PM -0300, Wander Lairson Costa wrote:
> On Mon, Apr 24, 2023 at 3:52 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > On Mon, Apr 24, 2023 at 03:43:09PM -0300, Wander Lairson Costa wrote:
> > > On Mon, Apr 24, 2023 at 3:09 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > > On Fri, Apr 14, 2023 at 09:55:28AM -0300, Wander Lairson Costa wrote:
> > > > > Due to the possibility of indirectly acquiring sleeping locks, it is
> > > > > unsafe to call put_task_struct() in atomic contexts when the kernel is
> > > > > compiled with PREEMPT_RT.
> > > > >
> > > > > To mitigate this issue, this commit introduces
> > > > > put_task_struct_atomic_safe(), which schedules __put_task_struct()
> > > > > through call_rcu() when PREEMPT_RT is enabled. While a workqueue would
> > > > > be a more natural approach, we cannot allocate dynamic memory from
> > > > > atomic context in PREEMPT_RT, making the code more complex.
> > > > >
> > > > > This implementation ensures safe execution in atomic contexts and
> > > > > avoids any potential issues that may arise from using the non-atomic
> > > > > version.
> > > > >
> > > > > Signed-off-by: Wander Lairson Costa <wander@redhat.com>
> > > > > Reported-by: Hu Chunyu <chuhu@redhat.com>
> > > > > Cc: Paul McKenney <paulmck@kernel.org>
> > > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > > ---
> > > > >  include/linux/sched/task.h | 31 +++++++++++++++++++++++++++++++
> > > > >  kernel/fork.c              |  8 ++++++++
> > > > >  2 files changed, 39 insertions(+)
> > > > >
> > > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > > > > index b597b97b1f8f..5c13b83d7008 100644
> > > > > --- a/include/linux/sched/task.h
> > > > > +++ b/include/linux/sched/task.h
> > > > > @@ -141,6 +141,37 @@ static inline void put_task_struct_many(struct task_struct *t, int nr)
> > > > >
> > > > >  void put_task_struct_rcu_user(struct task_struct *task);
> > > > >
> > > > > +extern void __delayed_put_task_struct(struct rcu_head *rhp);
> > > > > +
> > > > > +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> > > > > +{
> > > > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > > > > +             /*
> > > > > +              * Decrement the refcount explicitly to avoid unnecessarily
> > > > > +              * calling call_rcu.
> > > > > +              */
> > > > > +             if (refcount_dec_and_test(&task->usage))
> > > > > +                     /*
> > > > > +                      * under PREEMPT_RT, we can't call put_task_struct
> > > > > +                      * in atomic context because it will indirectly
> > > > > +                      * acquire sleeping locks.
> > > > > +                      * call_rcu() will schedule delayed_put_task_struct_rcu()
> > > > > +                      * to be called in process context.
> > > > > +                      *
> > > > > +                      * __put_task_struct() is called called when
> > > > > +                      * refcount_dec_and_test(&t->usage) succeeds.
> > > > > +                      *
> > > > > +                      * This means that it can't "conflict" with
> > > > > +                      * put_task_struct_rcu_user() which abuses ->rcu the same
> > > > > +                      * way; rcu_users has a reference so task->usage can't be
> > > > > +                      * zero after rcu_users 1 -> 0 transition.
> > > > > +                      */
> > > > > +                     call_rcu(&task->rcu, __delayed_put_task_struct);
> > > >
> > > > This will invoke __delayed_put_task_struct() with softirqs disabled.
> > > > Or do softirq-disabled contexts count as non-atomic in PREEMPT_RT?
> > >
> > > softirqs execute in thread context in PREEMPT_RT. We are good here.
> >
> > So the sleeping lock is a spinlock rather than (say) a mutex?
> 
> Yes, under PREEMPT_RT, spinlocks are implemented in terms of rtmutex.

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

                                                        Thanx, Paul

> > > > > +     } else {
> > > > > +             put_task_struct(task);
> > > > > +     }
> > > > > +}
> > > > > +
> > > > >  /* Free all architecture-specific resources held by a thread. */
> > > > >  void release_thread(struct task_struct *dead_task);
> > > > >
> > > > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > > > index 0c92f224c68c..9884794fe4b8 100644
> > > > > --- a/kernel/fork.c
> > > > > +++ b/kernel/fork.c
> > > > > @@ -854,6 +854,14 @@ void __put_task_struct(struct task_struct *tsk)
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(__put_task_struct);
> > > > >
> > > > > +void __delayed_put_task_struct(struct rcu_head *rhp)
> > > > > +{
> > > > > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > > > > +
> > > > > +     __put_task_struct(task);
> > > > > +}
> > > > > +EXPORT_SYMBOL_GPL(__delayed_put_task_struct);
> > > > > +
> > > > >  void __init __weak arch_task_cache_init(void) { }
> > > > >
> > > > >  /*
> > > > > --
> > > > > 2.39.2
> > > > >
> > > >
> > >
> >
>