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). 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
