Messages in this thread | | | Date | Mon, 13 Jul 2015 15:09:16 +0100 | From | Will Deacon <> | Subject | Re: [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
| |