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

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