Messages in this thread | | | Date | Mon, 13 Jul 2015 15:21:10 +0100 | From | Will Deacon <> | Subject | Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() |
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On Mon, Jul 13, 2015 at 03:09:15PM +0100, Will Deacon wrote: > 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.
I knew we'd had this conversation before ;)
http://lkml.kernel.org/r/20150120093443.GA11596@twins.programming.kicks-ass.net
Will
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