Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755146Ab0KKLAe (ORCPT ); Thu, 11 Nov 2010 06:00:34 -0500 Received: from mail.windriver.com ([147.11.1.11]:36984 "EHLO mail.windriver.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751358Ab0KKLAd (ORCPT ); Thu, 11 Nov 2010 06:00:33 -0500 Message-ID: <4CDBCCC8.4030104@windriver.com> Date: Thu, 11 Nov 2010 19:00:24 +0800 From: DDD User-Agent: Thunderbird 2.0.0.24 (X11/20100317) MIME-Version: 1.0 To: Ingo Molnar CC: dzickus@redhat.com, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86: avoid calling arch_trigger_all_cpu_backtrace() at the same time on SMP References: <1289442042-2515-1-git-send-email-dongdong.deng@windriver.com> <20101111092301.GD18789@elte.hu> <4CDBBCA5.50306@windriver.com> <20101111100109.GB24558@elte.hu> In-Reply-To: <20101111100109.GB24558@elte.hu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-OriginalArrivalTime: 11 Nov 2010 10:59:49.0766 (UTC) FILETIME=[8F7A6260:01CB818F] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1261 Lines: 39 Ingo Molnar wrote: > * DDD wrote: > >> Ingo Molnar wrote: >>> * Dongdong Deng wrote: >>> >>>> +static int backtrace_flag; >>>> + if (cmpxchg(&backtrace_flag, 0, 1) != 0) >>> Sorry to be a PITA, but i asked for test_and_set() because that's >>> the simplest primitive. cmpxchg() semantics is not nearly as >>> obvious and people regularly get it wrong :-/ >> As the 'backtrace_flag' could be accessed by multi-cpus on SMP at >> the same time, I use cmpxchg() for getting a atomic/memory barrier >> operation for 'backtrace_flag' variable. >> >> If we use test_and_set, maybe we need smp_wmb() after test_and_set. >> (If I wrong, please correct me, thanks. :-) ) > > No, test_and_set_bit() is SMP safe and is an implicit barrier as well - so no > smp_wmb() or other barriers are needed. Yep, the spin_lock of test_and_set_bit() could make sure that. Thank you very much, I will send out the new patch quickly. :-) Dongdong > > Thanks, > > Ingo > -- 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/