Re: [PATCH v3] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

["Luis Claudio R. Goncalves" <lgoncalv@redhat.com>]

On Thu, Apr 10, 2025 at 02:20:02PM +0200, Sebastian Andrzej Siewior wrote:
> On 2025-04-10 09:10:12 [-0300], Luis Claudio R. Goncalves wrote:
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -134,22 +134,12 @@ static inline void put_task_struct(struct task_struct *t)
> >  		return;
> >  
> >  	/*
> > -	 * In !RT, it is always safe to call __put_task_struct().
> > -	 * Under RT, we can only call it in preemptible context.
> > -	 */
> > -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
> > -		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;
> > -	}
> > -
> > -	/*
> > -	 * under PREEMPT_RT, we can't call put_task_struct
> > +	 * In !RT, it is always safe to call __put_task_struct(),
> > +	 * but under PREEMPT_RT, we can't call put_task_struct
> >  	 * in atomic context because it will indirectly
> > -	 * acquire sleeping locks.
> > +	 * acquire sleeping locks. The same is true if the
> > +	 * current process has a mutex enqueued (blocked on
> > +	 * a PI chain).
> >  	 *
> >  	 * call_rcu() will schedule delayed_put_task_struct_rcu()
> >  	 * to be called in process context.
> 
> Did you test it with lockdep with and without PREEMPT_RT? It would be
> nice to throw some testing on it.

I will re-run the full set of tests on both kernels.

> This comment here "call_rcu will schedule bla in process context" is
> wrong. It will schedule the callback in softirq context. Unless RCU is
> configured to run the callbacks in rcuc/ thread which is the default for
> PREEMPT_RT. Also delayed_put_task_struct_rcu() does not exist, imho
> never did.

I kept the original comment about the call_rcu in process context, but
didn't realize that wouldn't hold true for !RT. Would you prefer I adjust
the comments (for RT vs non-RT and other possibilities) or remove them
entirely?

And I completely missed delayed_put_task_struct_rcu() vs
__put_task_struct_rcu_cb() in the original comment.

Thank you again for the review!
Luis
> 
> Sebastian
> 
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