From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755313Ab2HAOvq (ORCPT ); Wed, 1 Aug 2012 10:51:46 -0400 Received: from www.linutronix.de ([62.245.132.108]:51505 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754992Ab2HAOvp (ORCPT ); Wed, 1 Aug 2012 10:51:45 -0400 Message-ID: <5019427B.1070002@linutronix.de> Date: Wed, 01 Aug 2012 16:51:39 +0200 From: Sebastian Andrzej Siewior User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.5) Gecko/20120624 Icedove/10.0.5 MIME-Version: 1.0 To: Oleg Nesterov CC: Roland McGrath , "H. Peter Anvin" , linux-kernel@vger.kernel.org, ananth@in.ibm.com, a.p.zijlstra@chello.nl, mingo@redhat.com, srikar@linux.vnet.ibm.com Subject: Re: Q: user_enable_single_step() && update_debugctlmsr() References: <1343735548-18101-2-git-send-email-bigeasy@linutronix.de> <20120731175108.GC14576@redhat.com> <50183273.9070304@linutronix.de> <20120801122616.GA32705@redhat.com> <20120801130118.GA2386@redhat.com> <50192FF5.1060208@linutronix.de> <20120801134652.GA4707@redhat.com> <50193528.3070503@linutronix.de> <20120801140121.GA5333@redhat.com> <50193B5C.90404@linutronix.de> <20120801143132.GA7550@redhat.com> In-Reply-To: <20120801143132.GA7550@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/01/2012 04:31 PM, Oleg Nesterov wrote: > And I think you missed my point. I'll try again. Okay. > We have the GDB process and the (stopped) tracee T. And we have > another task X which have TIF_SINGLESTEP but not TIF_BLOCKSTEP. > To simplify, suppose that X is already TASK_RUNNING but not on rq. > > GDB does ptrace(PTRACE_SINGLEBLOCK, T). This sets X->TIF_BLOCKSTEP. > Now suppose that GDB is preempted right after it does > update_debugctlmsr(), and the scheduler choses X as the next task. > > Both GDB and X do not have TIF_BLOCKSTEP, so __switch_to_extra() > does not update DEBUGCTLMSR_BTF. > > X returns to the user-mode with TIF_SINGLESTEP and TIF_BLOCKSTEP, > the latter is wrong. > > No? Yup, correct, you are right sir. Thank you for trying so hard to make me see this :) So a patch like --- a/arch/x86/kernel/step.c +++ b/arch/x86/kernel/step.c @@ -173,8 +173,8 @@ static void enable_step(struct task_struct *child, bool block) unsigned long debugctl = get_debugctlmsr(); debugctl |= DEBUGCTLMSR_BTF; - update_debugctlmsr(debugctl); set_tsk_thread_flag(child, TIF_BLOCKSTEP); + update_debugctlmsr(debugctl); } else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) { unsigned long debugctl = get_debugctlmsr(); should fix the race and _yes_ I also would follow your suggestion to remove this update_debugctlmsr() here since switch_to() should do this. > > Oleg. > Sebastian