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Message-ID: <Z_fWaT3UunvCFC1D@uudg.org>
Date: Thu, 10 Apr 2025 11:32:09 -0300
From: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Clark Williams <clrkwllms@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
David Vernet <dvernet@...a.com>, Barret Rhoden <brho@...gle.com>,
Josh Don <joshdon@...gle.com>, Crystal Wood <crwood@...hat.com>,
linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
Juri Lelli <juri.lelli@...hat.com>, Ben Segall <bsegall@...gle.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3] sched: do not call __put_task_struct() on rt if
pi_blocked_on is set
On Thu, Apr 10, 2025 at 02:40:59PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 10, 2025 at 09:10:12AM -0300, Luis Claudio R. Goncalves wrote:
> > With PREEMPT_RT enabled, some of the calls to put_task_struct() coming
> > from rt_mutex_adjust_prio_chain() could happen in preemptible context and
> > with a mutex enqueued. That could lead to this sequence:
> >
> > rt_mutex_adjust_prio_chain()
> > put_task_struct()
> > __put_task_struct()
> > sched_ext_free()
> > spin_lock_irqsave()
> > rtlock_lock() ---> TRIGGERS
> > lockdep_assert(!current->pi_blocked_on);
> >
> > Fix that by unconditionally resorting to the deferred call to
> > __put_task_struct().
> >
> > v2: (Rostedt) remove the #ifdef from put_task_struct() and create
> > tsk_is_pi_blocked_on() in sched.h to make the change cleaner.
> >
> > v3: (Sebastian and PeterZ) always call the RCU deferred __put_task_struct().
>
> Changelog goes below the --- line.
>
> > Suggested-by: Crystal Wood <crwood@...hat.com>
> > Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@...hat.com>
> > ---
> > include/linux/sched/task.h | 20 +++++---------------
> > 1 file changed, 5 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index 0f2aeb37bbb04..49847efe5559e 100644
> > --- 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;
> > - }
>
> I don't think you've substantiated why the !PREEMPT_RT case needs to go.
That was my misunderstanding of "unconditionally call the deferred
function". I see I took it too far and made the patch wrong.
I am testing v4 (closer to the original code with fixed comments) that is
basically:
if !RT
__put_task_struct (original code)
else
call_rcu(__put_task_struct_rcu_cb)
With the corrected comments Sebastian pointed out.
As soon as the tests complete I will post v4.
Thanks,
Luis
> > -
> > - /*
> > - * 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.
> > --
> > 2.49.0
> >
>
---end quoted text---
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