lkml.org 
[lkml]   [2015]   [Jul]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
On Mon, Jul 13, 2015 at 02:11:43PM +0100, Peter Zijlstra wrote:
> On Mon, Jul 13, 2015 at 01:15:04PM +0100, Will Deacon wrote:
> > smp_mb__after_unlock_lock is used to promote an UNLOCK + LOCK sequence
> > into a full memory barrier.
> >
> > However:
>
> > - The barrier only applies to UNLOCK + LOCK, not general
> > RELEASE + ACQUIRE operations
>
> No it does too; note that on ppc both acquire and release use lwsync and
> two lwsyncs do not make a sync.

Really? IIUC, that means smp_mb__after_unlock_lock needs to be a full
barrier on all architectures implementing smp_store_release as smp_mb() +
STORE, otherwise the following isn't ordered:

RELEASE X
smp_mb__after_unlock_lock()
ACQUIRE Y

On 32-bit ARM (at least), the ACQUIRE can be observed before the RELEASE.

Will


\
 
 \ /
  Last update: 2015-07-13 16:41    [W:0.557 / U:0.752 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site