On Mon, Jul 13, 2015 at 03:21:10PM +0100, Will Deacon wrote: > 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Ha! yes. And I had indeed forgotten about this argument. However I think we should look at the insides of the critical sections; for example (from Documentation/memory-barriers.txt): " *A = a; RELEASE M ACQUIRE N *B = b; could occur as: ACQUIRE N, STORE *B, STORE *A, RELEASE M" This could not in fact happen, even though we could flip M and N, A and B will remain strongly ordered. That said, I don't think this could even happen on PPC because we have load_acquire and store_release, this means that: *A = a lwsync store_release M load_acquire N lwsync *B = b And since the store to M is wrapped inside two lwsync there must be strong store order, and because the load from N is equally wrapped in two lwsyncs there must also be strong load order. In fact, no store/load can cross from before the first lwsync to after the latter and the other way around. So in that respect it does provide full load-store ordering. What it does not provide is order for M and N, nor does it provide transitivity, but looking at our documentation I'm not at all sure we guarantee that in any case. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
