From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755488Ab2HAPMY (ORCPT ); Wed, 1 Aug 2012 11:12:24 -0400 Received: from www.linutronix.de ([62.245.132.108]:51662 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755325Ab2HAPMX (ORCPT ); Wed, 1 Aug 2012 11:12:23 -0400 Message-ID: <50194750.2050601@linutronix.de> Date: Wed, 01 Aug 2012 17:12:16 +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: <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> <5019427B.1070002@linutronix.de> <20120801150119.GA8845@redhat.com> In-Reply-To: <20120801150119.GA8845@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 05:01 PM, Oleg Nesterov wrote: > On 08/01, Sebastian Andrzej Siewior wrote: >> 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 > > No, I don't think it can fix something ;) or make any difference. Why? You _first_ set the task flag followed by the CPU register. Now switch_to() would see the bit set and act. >> and _yes_ I also would follow your suggestion to >> remove this update_debugctlmsr() here since switch_to() should do this. > > Agreed, but once again, uprobes needs it if child == current (but we should > move this code into the trivial helper). If we change (I hope) uprobes to > avoid user_enable_single_step() we will export the helper. Okay. Looking at TIF_NOTSC I see that it does a preempt_disable() while playing with the bit. So this would be probably more obvious than switching the order :) > > Oleg. Sebastian