From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758829Ab1ELVz5 (ORCPT ); Thu, 12 May 2011 17:55:57 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:60065 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757968Ab1ELVzz (ORCPT ); Thu, 12 May 2011 17:55:55 -0400 Date: Thu, 12 May 2011 23:55:44 +0200 From: Ingo Molnar To: Frederic Weisbecker Cc: LKML , Peter Zijlstra , "H. Peter Anvin" , Thomas Gleixner , Steven Rostedt Subject: Re: [PATCH 2/2] x86: Make the x86-64 stacktrace code safely callable from scheduler Message-ID: <20110512215544.GC20939@elte.hu> References: <1305232326-9804-1-git-send-email-fweisbec@gmail.com> <1305232326-9804-3-git-send-email-fweisbec@gmail.com> <20110512212810.GA17596@elte.hu> <20110512214336.GE7410@nowhere> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110512214336.GE7410@nowhere> User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Frederic Weisbecker wrote: > On Thu, May 12, 2011 at 11:28:10PM +0200, Ingo Molnar wrote: > > > > * Frederic Weisbecker wrote: > > > > > Avoid potential scheduler recursion and deadlock from the > > > stacktrace code by avoiding rescheduling when we re-enable > > > preemption. > > > > > > This robustifies some scheduler trace events like sched switch > > > when they are used to produce callchains in perf or ftrace. > > > > > - put_cpu(); > > > + > > > + /* We want stacktrace to be computable anywhere, even in the scheduler */ > > > + preempt_enable_no_resched(); > > > > So what happens if a callchain profiling happens to be interrupted by a hardirq > > and the interrupt reschedules the current task? We'll miss the reschedule, > > right? > > > > preempt_enable_no_resched() is not a magic 'solve scheduler recursions' bullet > > - it's to be used only if something else will guarantee the preemption check! > > But nothing guarantees it here AFAICS. > > > > A better fix would be to use local_irq_save()/restore(). > > Good point, but then lockdep itself might trigger a stacktrace from local_irq_save, > leading to a stacktrace recursion. > > I can use raw_local_irq_disable(), or may be have a stacktrace recursion > protection. I fear the second solution could lead us to potentially lose > useful information if a stacktrace interrupts another one. Ok these are > extreme cases... i think raw_local_irq_disable() would be justified in this case. Thanks, Ingo