From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756407AbbBQNFh (ORCPT ); Tue, 17 Feb 2015 08:05:37 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:55497 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754960AbbBQNFg (ORCPT ); Tue, 17 Feb 2015 08:05:36 -0500 Date: Tue, 17 Feb 2015 14:05:23 +0100 From: Peter Zijlstra To: Kirill Tkhai Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Josh Poimboeuf , oleg@redhat.com, paulmck@linux.vnet.ibm.com Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles Message-ID: <20150217130523.GV24151@twins.programming.kicks-ass.net> References: <20150217104516.12144.85911.stgit@tkhai> <1424170021.5749.22.camel@tkhai> <20150217121258.GM5029@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150217121258.GM5029@twins.programming.kicks-ass.net> 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 Tue, Feb 17, 2015 at 01:12:58PM +0100, Peter Zijlstra wrote: > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -341,6 +341,22 @@ static struct rq *task_rq_lock(struct ta > raw_spin_lock_irqsave(&p->pi_lock, *flags); > rq = task_rq(p); > raw_spin_lock(&rq->lock); > + /* > + * move_queued_task() task_rq_lock() > + * > + * ACQUIRE (rq->lock) > + * [S] ->on_rq = MIGRATING [L] rq = task_rq() > + * WMB (__set_task_cpu()) ACQUIRE (rq->lock); > + * [S] ->cpu = new_cpu [L] task_rq() > + * [L] ->on_rq > + * RELEASE (rq->lock) > + * > + * If we observe the old cpu in task_rq_lock, the acquire of > + * the old rq->lock will fully serialize against the stores. > + * > + * If we observe the new cpu in task_rq_lock, the acquire will > + * pair with the WMB to ensure we must then also see migrating. > + */ > if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) > return rq; > raw_spin_unlock(&rq->lock); Hey Paul, remember this: https://lkml.org/lkml/2014/7/16/310 I just used a creative one :-) BTW, should we attempt to include that table in memory-barriers.txt like Mathieu said? As a cheat sheet with references to longer explanations for the 'interesting' ones? FWIW, we should probably update that table to include control dependencies too; we didn't (formally) have those back then I think. The blob under SMP BARRIER PAIRING does not mention pairing with control dependencies; and I'm rather sure I've done so.