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
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 <[email protected]>
Reported-by: Hu Chunyu <[email protected]>
Cc: Paul McKenney <[email protected]>
Cc: Thomas Gleixner <[email protected]>
---
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
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 <[email protected]>
Cc: Valentin Schneider <[email protected]>
---
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
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 <[email protected]>
Suggested-by: Sebastian Andrzej Siewior <[email protected]>
---
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
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 <[email protected]>
> Reported-by: Hu Chunyu <[email protected]>
> Cc: Paul McKenney <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> ---
> 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 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 <[email protected]>
> Cc: Valentin Schneider <[email protected]>
> ---
> 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
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 <[email protected]>
> > Reported-by: Hu Chunyu <[email protected]>
> > Cc: Paul McKenney <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > ---
> > 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 <[email protected]>
>>> Reported-by: Hu Chunyu <[email protected]>
>>> Cc: Paul McKenney <[email protected]>
>>> Cc: Thomas Gleixner <[email protected]>
>>> ---
>>> 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
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 <[email protected]>
> Reported-by: Hu Chunyu <[email protected]>
> Cc: Paul McKenney <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> ---
> 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 <[email protected]> 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 <[email protected]>
> > Reported-by: Hu Chunyu <[email protected]>
> > Cc: Paul McKenney <[email protected]>
> > Cc: Thomas Gleixner <[email protected]>
> > ---
> > 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 <[email protected]> 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 <[email protected]>
> > > Reported-by: Hu Chunyu <[email protected]>
> > > Cc: Paul McKenney <[email protected]>
> > > Cc: Thomas Gleixner <[email protected]>
> > > ---
> > > 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, 24 Apr 2023 11:52:52 -0700
"Paul E. McKenney" <[email protected]> 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 Mon, Apr 24, 2023 at 3:52 PM Paul E. McKenney <[email protected]> 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 <[email protected]> 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 <[email protected]>
> > > > Reported-by: Hu Chunyu <[email protected]>
> > > > Cc: Paul McKenney <[email protected]>
> > > > Cc: Thomas Gleixner <[email protected]>
> > > > ---
> > > > 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 <[email protected]> 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 <[email protected]> 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 <[email protected]>
> > > > > Reported-by: Hu Chunyu <[email protected]>
> > > > > Cc: Paul McKenney <[email protected]>
> > > > > Cc: Thomas Gleixner <[email protected]>
> > > > > ---
> > > > > 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 <[email protected]>
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
> > > > >
> > > >
> > >
> >
>