On Thu, Jan 28, 2016 at 10:46:23AM +0800, Boqun Feng wrote:
> On Wed, Jan 27, 2016 at 06:22:04PM +0000, Will Deacon wrote:
> > As much as we'd like to live in a world where RELEASE -> ACQUIRE is
> > always cheaply ordered and can be used to construct UNLOCK -> LOCK
> > definitions with similar guarantees, the grim reality is that this isn't
> > even possible on x86 (thanks to Paul for bringing us crashing down to
> > Earth).
> >
> > This patch handles the issue by introducing a new barrier macro,
> > smp_mb__after_release_acquire, that can be placed after an ACQUIRE that
> > either reads from a RELEASE or is in program-order after a RELEASE. The
> > barrier upgrades the RELEASE-ACQUIRE pair to a full memory barrier,
> > implying global transitivity. At the moment, it doesn't have any users,
> > so its existence serves mainly as a documentation aid and a potential
> > stepping stone to the reintroduction of smp_mb__after_unlock_lock() used
> > by RCU.
> >
> > Documentation/memory-barriers.txt is updated to describe more clearly
> > the ACQUIRE and RELEASE ordering in this area and to show some examples
> > of the new barrier in action.
> >
>
> Maybe also add an entry in "CPU MEMORY BARRIERS" section of
> memory-barriers.txt? Something like (copy and paste from you commit log
> ;-)):
>
> (*) smp_mb__after_release_acquire();
>
> Placed after an ACQUIRE that either reads from a RELEASE or is in
> program-order after a RELEASE. The barrier upgrades the
> RELEASE-ACQUIRE pair to a full memory barrier, implying global
> transitivity.
>
> This could give the readers an overview of the usage of this barrier.
Thanks, I'll add that.
> > diff --git a/arch/s390/include/asm/barrier.h b/arch/s390/include/asm/barrier.h
> > index 5c8db3ce61c8..ee31da604b11 100644
> > --- a/arch/s390/include/asm/barrier.h
> > +++ b/arch/s390/include/asm/barrier.h
> > @@ -45,6 +45,7 @@ do { \
> > ___p1; \
> > })
> >
> > +#define __smp_mb__release_acquire() __smp_mb()
>
> Should be __smp_mb__after_release_acquire(), so is the title of this
> patch ;-)
Well spotted. That's a hangover from v2, which I'll fix.
Cheers,
Will
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