From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755109Ab2HAOet (ORCPT ); Wed, 1 Aug 2012 10:34:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:54499 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754572Ab2HAOes (ORCPT ); Wed, 1 Aug 2012 10:34:48 -0400 Date: Wed, 1 Aug 2012 16:31:32 +0200 From: Oleg Nesterov To: Sebastian Andrzej Siewior 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() Message-ID: <20120801143132.GA7550@redhat.com> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50193B5C.90404@linutronix.de> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/01, Sebastian Andrzej Siewior wrote: > > On 08/01/2012 04:01 PM, Oleg Nesterov wrote: >> On 08/01, Sebastian Andrzej Siewior wrote: >>> >>> On 08/01/2012 03:46 PM, Oleg Nesterov wrote: >>> >>>>>> But, worse, isn't it wrong? Suppose that debugger switches to >>>>>> another TIF_SINGLESTEP&& !TIF_BLOCKSTEP task, in this case >>>>>> we "leak" DEBUGCTLMSR_BTF, no? >>>>> >>>>> __switch_to_xtra() should notice the difference in the TIF_BLOCKSTEP >>>>> flag and disable it. >>>> >>>> And how it can notice the difference if there is no difference? >>>> >>>> (unless, of course debugger is TIF_BLOCKSTEP'ed). >>> >>> Yes. enable_step() sets DEBUGCTLMSR_BTF along with TIF_BLOCKSTEP. >>> kprobes checks the same flag before touching DEBUGCTLMSR_BTF. >> >> It seems that you replied to the wrong email or I am confused ;) > > No I think I replied to the correct one :) > enable_step() is the only place for ptrace/debugger which is touching > DEBUGCTLMSR_BTF. It always sets DEBUGCTLMSR_BTF and TIF_BLOCKSTEP in > sync so why should they both end up different? And once > __switch_to_extra() notices that TIF_BLOCKSTEP from the previous task > is different from the next task is different, then the CPU flag has > to be changed. OK, I was confuse by "kprobes" above. And I think you missed my point. I'll try again. 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? Oleg.