Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751866AbaFBRZw (ORCPT ); Mon, 2 Jun 2014 13:25:52 -0400 Received: from g4t3425.houston.hp.com ([15.201.208.53]:24044 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750878AbaFBRZv (ORCPT ); Mon, 2 Jun 2014 13:25:51 -0400 Message-ID: <538CB389.5080108@hp.com> Date: Mon, 02 Jun 2014 13:25:29 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Jason Low CC: Mikulas Patocka , Linus Torvalds , Peter Zijlstra , jejb@parisc-linux.org, deller@gmx.de, John David Anglin , linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, chegu_vinod@hp.com, tglx@linutronix.de, riel@redhat.com, akpm@linux-foundation.org, davidlohr@hp.com, hpa@zytor.com, andi@firstfloor.org, aswin@hp.com, scott.norton@hp.com Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks References: <1401727810.7440.34.camel@j-VirtualBox> In-Reply-To: <1401727810.7440.34.camel@j-VirtualBox> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/02/2014 12:50 PM, Jason Low wrote: > On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote: >> If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at >> the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock, >> so, in this case, cmpxchg or xchg isn't really atomic at all. > So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at > the same time, would the below change address this problem? > > ----- > diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c > index 838dc9e..8396721 100644 > --- a/kernel/locking/mcs_spinlock.c > +++ b/kernel/locking/mcs_spinlock.c > @@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock) > if (likely(prev == NULL)) > return true; > > - ACCESS_ONCE(prev->next) = node; > + xchg(&prev->next, node); > > /* > * Normally @prev is untouchable after the above store; because at that > @@ -144,7 +144,7 @@ unqueue: > */ > > ACCESS_ONCE(next->prev) = prev; > - ACCESS_ONCE(prev->next) = next; > + xchg(&prev->next, next); > > return false; > } > > Doing an xchg is a very expensive operation compared with ACCESS_ONCE. I will not suggest doing that to make it right for PA-RISC at the expense of performance in other architectures. -Longman -- 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/