From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752679AbbKLOcb (ORCPT ); Thu, 12 Nov 2015 09:32:31 -0500 Received: from e38.co.us.ibm.com ([32.97.110.159]:46444 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750832AbbKLOca (ORCPT ); Thu, 12 Nov 2015 09:32:30 -0500 X-IBM-Helo: d03dlp03.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-kernel@vger.kernel.org Date: Thu, 12 Nov 2015 06:32:31 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: ralf@linux-mips.org, ddaney@caviumnetworks.com, linux-kernel@vger.kernel.org, Will Deacon , torvalds@linux-foundation.org, boqun.feng@gmail.com Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock() Message-ID: <20151112143231.GS3972@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20151112123123.GZ17308@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151112123123.GZ17308@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15111214-0029-0000-0000-00000E12D942 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 12, 2015 at 01:31:23PM +0100, Peter Zijlstra wrote: > Hi > > I think the MIPS arch_spin_unlock() is borken. > > spin_unlock() must have RELEASE semantics, these require that no LOADs > nor STOREs leak out from the critical section. > > >From what I know MIPS has a relaxed memory model which allows reads to > pass stores, and as implemented arch_spin_unlock() only issues a wmb > which doesn't order prior reads vs later stores. > > Therefore upgrade the wmb() to smp_mb(). > > (Also, why the unconditional wmb, as opposed to smp_wmb() ?) One guess is that they want to order I/O accesses within the critical section? Thanx, Paul > Maybe-Signed-off-by: Peter Zijlstra (Intel) > --- > diff --git a/arch/mips/include/asm/spinlock.h b/arch/mips/include/asm/spinlock.h > index 40196bebe849..b2ca13f06152 100644 > --- a/arch/mips/include/asm/spinlock.h > +++ b/arch/mips/include/asm/spinlock.h > @@ -140,7 +140,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock) > static inline void arch_spin_unlock(arch_spinlock_t *lock) > { > unsigned int serving_now = lock->h.serving_now + 1; > - wmb(); > + smp_mb(); > lock->h.serving_now = (u16)serving_now; > nudge_writes(); > } >