Re: [PATCH v3] barriers: introduce smp_mb__release_acquire and update documentation

[Peter Zijlstra]

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?