From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753583AbbKBN5e (ORCPT ); Mon, 2 Nov 2015 08:57:34 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:44446 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750856AbbKBN5d (ORCPT ); Mon, 2 Nov 2015 08:57:33 -0500 Date: Mon, 2 Nov 2015 14:57:26 +0100 From: Peter Zijlstra To: mingo@kernel.org, oleg@redhat.com Cc: linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, boqun.feng@gmail.com, corbet@lwn.net, mhocko@kernel.org, dhowells@redhat.com, torvalds@linux-foundation.org, will.deacon@arm.com Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire() Message-ID: <20151102135726.GR17308@twins.programming.kicks-ass.net> References: <20151102132901.157178466@infradead.org> <20151102134941.005198372@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151102134941.005198372@infradead.org> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 02, 2015 at 02:29:05PM +0100, Peter Zijlstra wrote: > Note that while smp_cond_acquire() has an explicit > smp_read_barrier_depends() for Alpha, neither sites it gets used in > were actually buggy on Alpha for their lack of it. The first uses > smp_rmb(), which on Alpha is a full barrier too and therefore serves > its purpose. The second had an explicit full barrier. > +/** > + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering > + * @cond: boolean expression to wait for > + * > + * Equivalent to using smp_load_acquire() on the condition variable but employs > + * the control dependency of the wait to reduce the barrier on many platforms. > + * > + * The control dependency provides a LOAD->STORE order, the additional RMB > + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order, > + * aka. ACQUIRE. > + */ > +#define smp_cond_acquire(cond) do { \ > + while (!(cond)) \ > + cpu_relax(); \ > + smp_read_barrier_depends(); /* ctrl */ \ > + smp_rmb(); /* ctrl + rmb := acquire */ \ > +} while (0) So per the above argument we could leave out the smp_read_barrier_depends() for Alpha, although that would break consistency with all the other control dependency primitives we have. It would avoid issuing a double barrier. Thoughts?