Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755050AbbLKMN2 (ORCPT ); Fri, 11 Dec 2015 07:13:28 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:59483 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754897AbbLKMN1 (ORCPT ); Fri, 11 Dec 2015 07:13:27 -0500 Date: Fri, 11 Dec 2015 13:13:19 +0100 From: Peter Zijlstra To: Will Deacon Cc: Andrew Pinski , Davidlohr Bueso , Thomas Gleixner , "Paul E. McKenney" , Ingo Molnar , Linux Kernel Mailing List , "linux-arm-kernel@lists.infradead.org" Subject: Re: FW: Commit 81a43adae3b9 (locking/mutex: Use acquire/release semantics) causing failures on arm64 (ThunderX) Message-ID: <20151211121319.GK6356@twins.programming.kicks-ass.net> References: <5669D5F2.5050004@caviumnetworks.com> <20151211084133.GE6356@twins.programming.kicks-ass.net> <20151211120419.GD18828@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151211120419.GD18828@arm.com> 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 Content-Length: 1569 Lines: 36 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. > 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/