Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754805AbbLKMR6 (ORCPT ); Fri, 11 Dec 2015 07:17:58 -0500 Received: from foss.arm.com ([217.140.101.70]:32992 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751937AbbLKMR5 (ORCPT ); Fri, 11 Dec 2015 07:17:57 -0500 Date: Fri, 11 Dec 2015 12:18:00 +0000 From: Will Deacon To: Peter Zijlstra Cc: Andrew Pinski , Davidlohr Bueso , Thomas Gleixner , "Paul E. McKenney" , Ingo Molnar , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" , david.daney@cavium.com Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) Message-ID: <20151211121759.GE18828@arm.com> References: <5669D5F2.5050004@caviumnetworks.com> <20151211084133.GE6356@twins.programming.kicks-ass.net> <20151211120419.GD18828@arm.com> <20151211121319.GK6356@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151211121319.GK6356@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1823 Lines: 43 On Fri, Dec 11, 2015 at 01:13:19PM +0100, Peter Zijlstra wrote: > On Fri, Dec 11, 2015 at 12:04:19PM +0000, Will Deacon wrote: > > I think Andrew meant the atomic_xchg_acquire at the start of osq_lock, > > as opposed to "compare and swap". In which case, it does look like > > there's a bug here because there is nothing to order the initialisation > > of the node fields with publishing of the node, whether that's > > indirectly as a result of setting the tail to the current CPU or > > directly as a result of the WRITE_ONCE. > > Agreed, this does indeed look like a bug. If confirmed please write a > shiny changelog and I'll queue asap. Yup. I've failed to reproduce the issue locally, so we'll need to wait for Andrew and/or David to get back to us first. Will > > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c > > index d092a0c9c2d4..05a37857ab55 100644 > > --- a/kernel/locking/osq_lock.c > > +++ b/kernel/locking/osq_lock.c > > @@ -93,10 +93,12 @@ bool osq_lock(struct optimistic_spin_queue *lock) > > node->cpu = curr; > > > > /* > > - * ACQUIRE semantics, pairs with corresponding RELEASE > > - * in unlock() uncontended, or fastpath. > > + * We need both ACQUIRE (pairs with corresponding RELEASE in > > + * unlock() uncontended, or fastpath) and RELEASE (to publish > > + * the node fields we just initialised) semantics when updating > > + * the lock tail. > > */ > > - old = atomic_xchg_acquire(&lock->tail, curr); > > + old = atomic_xchg(&lock->tail, curr); > > if (old == OSQ_UNLOCKED_VAL) > > return true; > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/