On Wed, Jan 27, 2016 at 03:00:11PM -0800, Paul E. McKenney wrote: > On Wed, Jan 27, 2016 at 06:39:23PM +0000, Will Deacon wrote: > > On Wed, Jan 27, 2016 at 07:25:51PM +0100, Peter Zijlstra 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. > > > > > > So the obvious question is: do we have a use-case? > > > > We have a use-case for smp_mb__after_unlock_lock, so I think we should > > either strengthen our locking guarantees so that smp_mb__after_unlock_lock > > isn't needed or introduce smp_mb__after_release_acquire to close the gap. > > As it stands, we've got an inconsistency (despite it being hidden inside > > RCU). > > > > The main advantage of this patch is a documentation aid, in my opinion > > (hell, we talk about smp_mb__after_unlock_lock already when reasoning > > about this stuff). > > But wasn't there an x86 potential use case that required placing the > strengthening macro after the release and before the acquire? Or is > this a case of old age striking again? The proposal here doesn't order the release/acquire operations with each other -- it just says that they combine with smp_mb__after_release_acquire() to make a full barrier, so I don't think it should matter for the intra-thread case, which is the one that x86 cares out iiuc. Will -- 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
