linux-kernel - Re: [PATCH v2] barriers: introduce smp_mb__release

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Message-ID: <20151007111915.GF17308@twins.programming.kicks-ass.net>
Date:	Wed, 7 Oct 2015 13:19:15 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] barriers: introduce smp_mb__release_acquire and
 update documentation

On Wed, Oct 07, 2015 at 11:59:28AM +0100, 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__release_acquire, that can be placed between a RELEASE and a
> subsequent ACQUIRE operation in order to upgrade them to a full memory
> barrier. At the moment, it doesn't have any users, so its existence
> serves mainly as a documentation aid.

Does we want to go revert 12d560f4ea87 ("rcu,locking: Privatize
smp_mb__after_unlock_lock()") for that same reason?

> Documentation/memory-barriers.txt is updated to describe more clearly
> the ACQUIRE and RELEASE ordering in this area and to show an example of
> the new barrier in action.

The only nit I have is that if we revert the above it might be make
sense to more clearly call out the distinction between the two.
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