From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755482AbYAJVlL (ORCPT ); Thu, 10 Jan 2008 16:41:11 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753245AbYAJVk5 (ORCPT ); Thu, 10 Jan 2008 16:40:57 -0500 Received: from x346.tv-sign.ru ([89.108.83.215]:59815 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753184AbYAJVk5 (ORCPT ); Thu, 10 Jan 2008 16:40:57 -0500 Date: Fri, 11 Jan 2008 00:39:32 +0300 From: Oleg Nesterov To: Petr Tesarik Cc: "Eric W. Biederman" , Andrew Morton , Davide Libenzi , Ingo Molnar , Linus Torvalds , Roland McGrath , linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] ptrace_stop: remove the wrong ->group_stop_count bookkeeping Message-ID: <20080110213932.GA3396@tv-sign.ru> References: <20071208183801.GA9943@tv-sign.ru> <20071209140525.GA131@tv-sign.ru> <4785F675.3070709@suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4785F675.3070709@suse.cz> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/10, Petr Tesarik wrote: > > I can actually see a bug which may be related: > > 1. a process creates a thread (or more threads) > 2. I attach/detach to that thread with strace several times > (each time pressing CTRL-C to quit strace) > 3. the whole thread group (except the traced thread) ends in > TASK_STOPPED > > I looked at what strace was doing to that thread, and it sometimes sends > SIGSTOP shortly before detaching. This is done when the thread is > running, i.e. not waiting in ptrace_stop. Then PTRACE_DETACH returns > - -ESRCH because it requires the tracee to be stopped -- just like all > PTRACE_* requests except TRACEME and ATTACH. So, strace has no other > option than to send an explicit SIGSTOP to the thread to stop it and > discard it afterwards. > > Could this be related? Perhaps yes. But there are so many oddities in this area. I don't know what really happens with your test-case, but afaics this can happen even without ptrace_stop() playing with the group stop. Let's suppose that strace detached all sub-threads except T which is running, and now strace does ptrace(PTRACE_DETACH, T). This fails, so strace does kill(T, SIGSTOP). Note that it use kill(), not tkill(). This means another sub-thread can dequeue this signal and initiate the group stop (remember, it was already detached and thus it is not traced any longer). Now strace does wait4(T, __WALL). T notices the group stop in progress, calls handle_group_stop(), and notifies its parent - strace. wait4() returns success, strace does ptrace(PTRACE_DETACH, T) again. Now T is TASK_STOPPED, ptrace() changes the state to TASK_TRACED and finally does ptrace_untrace(). ptrace_untrace() sees TASK_TRACED. But it is possible that the group stop is not completed yet (some sub-thread didn't pass handle_group_stop()), in that case we are doing signal_wake_up(T, 1) so it becomes running. I still think this series makes sense even if not complete. Oleg.