Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
locks. Therefore, it can't be called from an non-preemptible context.

One practical example is splat inside inactive_task_timer(), which is
called in a interrupt context:

CPU: 1 PID: 2848 Comm: life Kdump: loaded Tainted: G W ---------
 Hardware name: HP ProLiant DL388p Gen8, BIOS P70 07/15/2012
 Call Trace:
 dump_stack_lvl+0x57/0x7d
 mark_lock_irq.cold+0x33/0xba
 ? stack_trace_save+0x4b/0x70
 ? save_trace+0x55/0x150
 mark_lock+0x1e7/0x400
 mark_usage+0x11d/0x140
 __lock_acquire+0x30d/0x930
 lock_acquire.part.0+0x9c/0x210
 ? refill_obj_stock+0x3d/0x3a0
 ? rcu_read_lock_sched_held+0x3f/0x70
 ? trace_lock_acquire+0x38/0x140
 ? lock_acquire+0x30/0x80
 ? refill_obj_stock+0x3d/0x3a0
 rt_spin_lock+0x27/0xe0
 ? refill_obj_stock+0x3d/0x3a0
 refill_obj_stock+0x3d/0x3a0
 ? inactive_task_timer+0x1ad/0x340
 kmem_cache_free+0x357/0x560
 inactive_task_timer+0x1ad/0x340
 ? switched_from_dl+0x2d0/0x2d0
 __run_hrtimer+0x8a/0x1a0
 __hrtimer_run_queues+0x91/0x130
 hrtimer_interrupt+0x10f/0x220
 __sysvec_apic_timer_interrupt+0x7b/0xd0
 sysvec_apic_timer_interrupt+0x4f/0xd0
 ? asm_sysvec_apic_timer_interrupt+0xa/0x20
 asm_sysvec_apic_timer_interrupt+0x12/0x20
 RIP: 0033:0x7fff196bf6f5

Instead of calling __put_task_struct() directly, we defer it using
call_rcu(). A more natural approach would use a workqueue, but since
in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
the code would become more complex because we would need to put the
work_struct instance in the task_struct and initialize it when we
allocate a new task_struct.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Reported-by: Hu Chunyu <chuhu@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Valentin Schneider <vschneid@redhat.com>
Cc: Paul McKenney <paulmck@kernel.org>
---
 kernel/fork.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 9f7fe3541897..b2d0d62c9b9d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -840,7 +840,7 @@ static inline void put_signal_struct(struct signal_struct *sig)
 		free_signal_struct(sig);
 }
 
-void __put_task_struct(struct task_struct *tsk)
+static void ___put_task_struct(struct task_struct *tsk)
 {
 	WARN_ON(!tsk->exit_state);
 	WARN_ON(refcount_read(&tsk->usage));
@@ -857,6 +857,26 @@ void __put_task_struct(struct task_struct *tsk)
 	sched_core_free(tsk);
 	free_task(tsk);
 }
+
+static void __delayed_put_task_struct(struct rcu_head *rhp)
+{
+	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
+
+	___put_task_struct(task);
+}
+
+void __put_task_struct(struct task_struct *tsk)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
+		/*
+		 * under PREEMPT_RT, we can't call put_task_struct
+		 * in atomic context because it will indirectly
+		 * acquire sleeping locks.
+		 */
+		call_rcu(&tsk->rcu, __delayed_put_task_struct);
+	else
+		___put_task_struct(tsk);
+}
 EXPORT_SYMBOL_GPL(__put_task_struct);
 
 void __init __weak arch_task_cache_init(void) { }
-- 
2.39.1

On 02/01, Wander Lairson Costa wrote:
>
> Instead of calling __put_task_struct() directly, we defer it using
> call_rcu(). A more natural approach would use a workqueue, but since
> in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
> the code would become more complex because we would need to put the
> work_struct instance in the task_struct and initialize it when we
> allocate a new task_struct.

I don't think I can ack the changes in PREEMPT_RT but this version LGTM.




just a couple of purely cosmetic nits, feel free to ignore...

> +static void __delayed_put_task_struct(struct rcu_head *rhp)
> +{
> +	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> +
> +	___put_task_struct(task);
> +}

We already have delayed_put_task_struct() which differs very much.
Perhaps something like ___put_task_struct() will look less confusing.

> +void __put_task_struct(struct task_struct *tsk)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> +		/*
> +		 * under PREEMPT_RT, we can't call put_task_struct
> +		 * in atomic context because it will indirectly
> +		 * acquire sleeping locks.
> +		 */
> +		call_rcu(&tsk->rcu, __delayed_put_task_struct);

Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
union. May be this is obvious, but I was confused when I looked at the previous
version ;)

but again, feel free to ignore.

Oleg.

On Thu, Feb 2, 2023 at 3:37 PM, Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 02/01, Wander Lairson Costa wrote:
> >
> > Instead of calling __put_task_struct() directly, we defer it using
> > call_rcu(). A more natural approach would use a workqueue, but since
> > in PREEMPT_RT, we can't allocate dynamic memory from atomic context,
> > the code would become more complex because we would need to put the
> > work_struct instance in the task_struct and initialize it when we
> > allocate a new task_struct.
>
> I don't think I can ack the changes in PREEMPT_RT but this version LGTM.
>
>
>
>
> just a couple of purely cosmetic nits, feel free to ignore...
>
> > +static void __delayed_put_task_struct(struct rcu_head *rhp)
> > +{
> > +     struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > +     ___put_task_struct(task);
> > +}
>
> We already have delayed_put_task_struct() which differs very much.
> Perhaps something like ___put_task_struct() will look less confusing.
>

___put_task_struct()? I already added a function with this name below.

> > +void __put_task_struct(struct task_struct *tsk)
> > +{
> > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> > +             /*
> > +              * under PREEMPT_RT, we can't call put_task_struct
> > +              * in atomic context because it will indirectly
> > +              * acquire sleeping locks.
> > +              */
> > +             call_rcu(&tsk->rcu, __delayed_put_task_struct);
>
> Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
> union. May be this is obvious, but I was confused when I looked at the previous
> version ;)
>

Makes sense, I will add it in the next version.

> but again, feel free to ignore.
>
> Oleg.
>

On 02/02, Wander Lairson Costa wrote:
>
> > We already have delayed_put_task_struct() which differs very much.
> > Perhaps something like ___put_task_struct() will look less confusing.
> >
>
> ___put_task_struct()? I already added a function with this name below.

Ah, I meant ___put_task_struct_rcu() or something like this. Bug again
this is just cosmetic nit, please ignore

> > > +void __put_task_struct(struct task_struct *tsk)
> > > +{
> > > +     if (IS_ENABLED(CONFIG_PREEMPT_RT) && (!preemptible() || !in_task()))
> > > +             /*
> > > +              * under PREEMPT_RT, we can't call put_task_struct
> > > +              * in atomic context because it will indirectly
> > > +              * acquire sleeping locks.
> > > +              */
> > > +             call_rcu(&tsk->rcu, __delayed_put_task_struct);
> >
> > Perhaps this deserves additional note to explain why is it safe to use tsk->rcu
> > union. May be this is obvious, but I was confused when I looked at the previous
> > version ;)
> >
>
> Makes sense, I will add it in the next version.

Thanks ;)

Oleg.