From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v3] barriers: introduce smp_mb__release_acquire and update documentation Date: Wed, 27 Jan 2016 19:25:51 +0100 Message-ID: <20160127182551.GX6357@twins.programming.kicks-ass.net> References: <1453918924-27606-1-git-send-email-will.deacon@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from casper.infradead.org ([85.118.1.10]:57550 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934590AbcA0SZy (ORCPT ); Wed, 27 Jan 2016 13:25:54 -0500 Content-Disposition: inline In-Reply-To: <1453918924-27606-1-git-send-email-will.deacon@arm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Will Deacon Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Boqun Feng , "Paul E. McKenney" 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?