Re: [PATCH] rtmutex: Add acquire semantics for rtmutex lock acquisition

[Mel Gorman]

On Fri, Dec 16, 2022 at 11:14:12AM +0000, Will Deacon wrote:
> > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> > index 35212f260148..af0dbe4d5e97 100644
> > --- a/kernel/locking/rtmutex.c
> > +++ b/kernel/locking/rtmutex.c
> > @@ -238,6 +238,13 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
> >  		owner = *p;
> >  	} while (cmpxchg_relaxed(p, owner,
> >  				 owner | RT_MUTEX_HAS_WAITERS) != owner);
> > +
> > +	/*
> > +	 * The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE
> > +	 * operations in the event of contention. Ensure the successful
> > +	 * cmpxchg is visible.
> > +	 */
> > +	smp_mb__after_atomic();
> 
> Could we use smp_acquire__after_ctrl_dep() instead?
> 

It's might be sufficient but it's not as obviously correct. It lacks an
obvious pairing that is definitely correct but the control dependency
combined with the smp_acquire__after_ctrl_dep *should* be sufficient
against the lock fast path based on the available documentation. However,
I was unable to convince myself that this is definitely correct on all CPUs.

Given that arm64 was trivial to crash on PREEMPT_RT before the patch
(hackbench pipes also triggers the same problem), I'm reluctant to try and
be too clever particularly as I didn't have a reproducer for a problem in
this specific path. If someone can demonstrate a reasonable case where
smp_mb__after_atomic() is too heavy and document that it can be safely
relaxed then great. At least if that relaxing is wrong, there will be a
bisection point between the fix and the reintroduction of damage.

-- 
Mel Gorman
SUSE Labs