Re: [PATCH] Document that wake_up(), complete() and co. imply a full memory barrier

Ingo Molnar
2009-04-22T15:20:13Z

* David Howells <dhowells@redhat.com> wrote:

> Ingo Molnar <mingo@elte.hu> wrote:
>
> > No. They dont generally imply a full memory barrier versus any
> > arbitrary prior (or following) memory access.
> >
> > try_to_wake_up() has an smp_wmb() so it is a write memory barrier
> > (but not necessarily a read memory barrier). Otherwise there are
> > spinlocks there but spinlocks are not explicit 'full memory
> > barriers'.
>
> Blech. That's a good point LOCK...UNLOCK does not imply a full
> barrier.
>
> So we can't assume that complete(), wake_up() and co. imply any
> barriers.
>
> All we can assume is that try_to_wake_up() implies a write
> barrier, but we can't assume that that will be called via
> __wake_up_common().

Yeah, it's all too special-case. We might rely on it in special,
well-argued cases but we should not document it as a general barrier
property.

Ingo