Re: [PATCH v3] mm/slab: don't use kfree_rcu sheaves on PREEMPT_RT in kvfree_call_rcu()

[Harry Yoo <harry@kernel.org>]

On Mon, Aug 31, 2026 at 04:35:00PM +0200, Sebastian Andrzej Siewior wrote:
> On 2026-08-31 20:32:22 [+0700], ThangNN99 wrote:
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -2034,7 +2027,14 @@ void kvfree_call_rcu(struct kvfree_rcu_head *head, void *ptr)
> >  	if (!head)
> >  		might_sleep();
> >  
> > -	if (kfree_rcu_sheaf(ptr))
> > +	/*
> > +	 * Callers may hold a raw_spinlock_t here on PREEMPT_RT (e.g.
> > +	 * set_cpus_allowed_force(), whose callers all hold
> > +	 * task_struct::pi_lock), and the sheaf/barn locks are also taken
> > +	 * as blocking locks elsewhere, so trying them here creates a
> > +	 * lockdep-visible ordering conflict. Skip sheaves on PREEMPT_RT.
> > +	 */
> 
> You mix up things. A raw_spinlock_t should work in general and should
> not cause a problem. The task_struct::pi_lock is special: It used during
> wakes and the inner waitlock of the rtmutex acquires it even during a
> trylock. For PREEMPT_RT we don't want to acquire any locks while the
> pi_lock is held.

So... even can_spin_trylock() is not enough handle the pi_lock's
special case.

Probably we should make can_spin_trylock() check if current tasks's
pi_lock is acquired, to determine if it's safe to call _trylock()?

> What about
>   
>     kvfree_rcu() is called by set_cpus_allowed_force() with
>     task_struct::pi_lock acquired. On PREEMPT_RT the local_trylock()
>     usage below will acquire the waitlock which must be avoided.
>     Therefore avoid it on PREEMPT_RT.

-- 
Cheers,
Harry / Hyeonggon