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) { }
/*
--
2.39.2On 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
>
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
> >
>
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
> > >
> >
>
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
> > > >
> > >
> >
>
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
> > > > >
> > > >
> > >
> >
>
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
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
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
>
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
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