Re: [RESEND PATCH v4] sched: do not call __put_task_struct() on rt if pi_blocked_on is set

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

On Tue, Jun 17, 2025 at 11:26:09AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 13, 2025 at 12:05:14PM -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() if PREEMPT_RT is enabled.
> > 
> 
> Should this have a Fixes: tag and go into /urgent?

Makes sense! I will add the tag:

Fixes: 893cdaaa3977b ("sched: avoid false lockdep splat in put_task_struct()")

and resend.

Thank you!
 
> > Suggested-by: Crystal Wood <crwood@redhat.com>
> > Signed-off-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
> > ---
> > 
> > Resent as a gentle reminder, because this issue results in scary backtraces,
> > not obvious to debug and pinpoint root cause.
> > 
> > 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, PeterZ) always call the deferred __put_task_struct() on RT.
> > v4: Fix the implementation of what was requested on v3.
> > 
> >  include/linux/sched/task.h |   17 ++++++++---------
> >  1 file changed, 8 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
> > index 0f2aeb37bbb04..51678a541477a 100644
> > --- a/include/linux/sched/task.h
> > +++ b/include/linux/sched/task.h
> > @@ -134,11 +134,8 @@ 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().
> > -	 * Under RT, we can only call it in preemptible context.
> > -	 */
> > -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
> > +	/* 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);
> > @@ -148,11 +145,13 @@ static inline void put_task_struct(struct task_struct *t)
> >  	}
> >  
> >  	/*
> > -	 * under PREEMPT_RT, we can't call put_task_struct
> > +	 * 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()
> > +	 * call_rcu() will schedule __put_task_struct_rcu_cb()
> >  	 * to be called in process context.
> >  	 *
> >  	 * __put_task_struct() is called when
> > @@ -165,7 +164,7 @@ static inline void put_task_struct(struct task_struct *t)
> >  	 *
> >  	 * delayed_free_task() also uses ->rcu, but it is only called
> >  	 * when it fails to fork a process. Therefore, there is no
> > -	 * way it can conflict with put_task_struct().
> > +	 * way it can conflict with __put_task_struct().
> >  	 */
> >  	call_rcu(&t->rcu, __put_task_struct_rcu_cb);
> >  }
> > 
> > ----- End forwarded message -----
> > 
> 
---end quoted text---