Re: [PATCH] futex: Fix might_sleep() warning in futex_pivot_pending()

[Peter Zijlstra]

On Thu, Aug 20, 2026 at 03:32:17PM +0800, Yao Kai wrote:

> > I still think we should use smp_mb() here, smp_mb__after_spinlock() only
> > orders accesses preceding the lock acquisition against later accesses. The
> > waitqueue insertion happens after that acquisition, so I don't think
> > smp_mb__after_spinlock() covers it here.
> > 
> 
> On further thought, please disregard my previous objection to
> smp_mb__after_spinlock().
> 
> I was considering the documented semantics of
> smp_mb__after_spinlock() in isolation and overlooked that the full
> waiter-side sequence also includes the subsequent mutex acquisition in
> futex_pivot_pending():
> 
>         STORE waitqueue entry
>         UNLOCK wq_head->lock
>         smp_mb__after_spinlock()
>         LOCK mmph->lock
>         LOAD refcount
> 
> On architectures where the UNLOCK+LOCK sequence needs strengthening,
> smp_mb__after_spinlock() provides the required full barrier. On
> architectures where it is a no-op, the lock acquisition is already
> strong enough to provide the required ordering.
> 
> So your version looks sufficient. Sorry for the noise.

No problem.