Re: [PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

[Luis Claudio R. Goncalves]

On Tue, Jul 29, 2025 at 01:47:03PM +0200, Oleg Nesterov wrote:
> On 07/29, Luis Claudio R. Goncalves wrote:
> >
> > From: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
> > Subject: sched: restore the behavior of put_task_struct() for non-rt
> >
> > Commit 8671bad873eb ("sched: Do not call __put_task_struct() on rt
> > if pi_blocked_on is set") changed the behavior of put_task_struct()
> > unconditionally, even when PREEMPT_RT was not enabled, in clear mismatch
> > with the commit description.
> >
> > Restore the previous behavior of put_task_struct() for the PREEMPT_RT
> > disabled case.
> >
> > Fixes: 8671bad873eb ("sched: Do not call __put_task_struct() on rt if pi_blocked_on is set")
> > Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@xxxxxxxxxx>
> > ---
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index ea41795a352b..51678a541477 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -130,6 +133,16 @@ static inline void put_task_struct(struct task_struct *t)
> >  	if (!refcount_dec_and_test(&t->usage))
> >  		return;
> >
> > +	/* In !RT, it is always safe to call __put_task_struct(). */
> > +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
> > +
> > +		lock_map_acquire_try(&put_task_map);
> > +		__put_task_struct(t);
> > +		lock_map_release(&put_task_map);
> > +		return;
> > +	}
> 
> FWIW:
> 
> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> 
> 
> At the same time... I don't understand this DEFINE_WAIT_OVERRIDE_MAP().
> IIUC, we need to shut up lockdep when put_task_struct() is called under
> raw_spinlock_t and __put_task_struct() paths take spinlock_t, right?
> Perhaps this deserves a comment...

I reverted that code to the previous state, commit 893cdaaa3977 ("sched:
avoid false lockdep splat in put_task_struct()") and simplified the "if"
statement. In the original code, PREEMPT_RT could call __put_task_struct()
if the context was preemptible. But in the proposed code __put_task_struct()
is only called if PREEMPT_RT is disabled. In this case I believe we could
simply do:

+	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
+		__put_task_struct(t);
+		return;
+	}

Does that make sense?

Luis
 
> But if I am right, why LD_WAIT_SLEEP? LD_WAIT_CONFIG should equally work, no?
> 
> LD_WAIT_SLEEP can fool lockdep more than we need, suppose that __put_task_struct()
> does mutex_lock(). Not really a problem, might_sleep/etc will complain in this
> case, but still.
> 
> Or I am totally confused?
> 
> Oleg.
> 
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