From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932727AbdBVODa (ORCPT ); Wed, 22 Feb 2017 09:03:30 -0500 Received: from bombadil.infradead.org ([65.50.211.133]:55756 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932360AbdBVODW (ORCPT ); Wed, 22 Feb 2017 09:03:22 -0500 Date: Wed, 22 Feb 2017 15:03:16 +0100 From: Peter Zijlstra To: Darren Hart Cc: tglx@linutronix.de, mingo@kernel.org, juri.lelli@arm.com, rostedt@goodmis.org, xlpang@redhat.com, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, mathieu.desnoyers@efficios.com, jdesfossez@efficios.com, bristot@redhat.com, paulmck@linux.vnet.ibm.com Subject: Re: [PATCH -v4 04/10] futex: Use smp_store_release() in mark_wake_futex() Message-ID: <20170222140316.GT6515@twins.programming.kicks-ass.net> References: <20161213083638.938898295@infradead.org> <20161213085730.289714424@infradead.org> <20161217005045.GG62123@f23x64.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20161217005045.GG62123@f23x64.localdomain> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 16, 2016 at 04:50:45PM -0800, Darren Hart wrote: > On Tue, Dec 13, 2016 at 09:36:42AM +0100, Peter Zijlstra wrote: > > Since the futex_q can dissapear the instruction after assigning NULL, > > this really should be a RELEASE barrier. That stops loads from hitting > > dead memory too. > > > > +Paul McKenney > > Per the introduction of the comment below from: > > f1a11e0 futex: remove the wait queue > > I believe the intent was to ensure the plist_del in ... the previous > __unqueue_futex(q) ... from getting ahead of the smp_store_release added here, > which could result in q being destroyed by the waking task before plist_del can > act on it. Is that > right? > > The comment below predates the refactoring which hid plist_del under the > __unqueue_futex() making it a bit less clear as to the associated plist_del: > > However, since this comment, we have moved the wake-up out of wake_futex through > the use of wake queues (wake_up_q) which now happens after the hb lock is > released (see futex_wake, futex_wake_op, and futex_requeue). Is this race still > a valid concern? Yes I think so, since __unqueue_futex() dereferences lock_ptr and does stores in the memory it points to, those stores must not happen _after_ we NULL lock_ptr itself. futex_wait(), which calls unqueue_me() could have had a spurious wakeup and observe our NULL store and 'free' the futex_q.