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

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 16, 2015 at 11:29:08AM +0100, Will Deacon wrote:
> > Indeed, that is a hole in the definition, that I think we should close.

> I'm struggling to understand the hole, but here's my intuition. If an
> ACQUIRE on CPUx reads from a RELEASE by CPUy, then I'd expect CPUx to
> observe all memory accessed performed by CPUy prior to the RELEASE
> before it observes the RELEASE itself, regardless of this new barrier.
> I think this matches what we currently have in memory-barriers.txt (i.e.
> acquire/release are neither transitive or multi-copy atomic).

Ah agreed. I seem to have gotten my brain in a tangle.

Basically where a program order release+acquire relies on an address
dependency, a cross cpu release+acquire relies on causality. If we
observe the release, we must also observe everything prior to it etc.