Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760141AbXHXUX4 (ORCPT ); Fri, 24 Aug 2007 16:23:56 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759149AbXHXUXp (ORCPT ); Fri, 24 Aug 2007 16:23:45 -0400 Received: from x346.tv-sign.ru ([89.108.83.215]:39514 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753096AbXHXUXo (ORCPT ); Fri, 24 Aug 2007 16:23:44 -0400 Date: Sat, 25 Aug 2007 00:23:05 +0400 From: Oleg Nesterov To: Sukadev Bhattiprolu Cc: taoyue , Andrew Morton , Alexey Dobriyan , Ingo Molnar , Thomas Gleixner , Roland McGrath , linux-kernel@vger.kernel.org, stable@kernel.org Subject: Re: [PATCH] sigqueue_free: fix the race with collect_signal() Message-ID: <20070824202305.GA274@tv-sign.ru> References: <20070823134538.GA1358@tv-sign.ru> <46CEEA94.2070902@windriver.com> <20070824074558.GA86@tv-sign.ru> <46CF4DCB.6030304@windriver.com> <20070824110836.GA74@tv-sign.ru> <46CF3988.1020408@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46CF3988.1020408@us.ibm.com> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2862 Lines: 71 On 08/24, Sukadev Bhattiprolu wrote: > > Oleg Nesterov wrote: > >On 08/24, taoyue wrote: > > > >>Oleg Nesterov wrote: > >> > >>>>collect_signal: sigqueue_free: > >>>> > >>>> list_del_init(&first->list); > >>>> spin_lock_irqsave(lock, flags); > >>>> > >>>> > >>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > >>> > >>> > >>>> if (!list_empty(&q->list)) > >>>> list_del_init(&q->list); > >>>> spin_unlock_irqrestore(lock, > >>>> flags); > >>>> q->flags &= ~SIGQUEUE_PREALLOC; > >>>> > >>>> __sigqueue_free(first); __sigqueue_free(q); > >>>> > >>>> > >>>collect_signal() is always called under ->siglock which is also taken by > >>>sigqueue_free(), so this is not possible. > >>> > >>> > >>> > >>I know, using current->sighand->siglock to prevent one sigqueue > >>is free twice. I want to know whether it is possible that the two > >>function is called in different thread. If that, the spin_lock is useless. > >> > > > >Not sure I understand. Yes, it is possible they are called by 2 different > >threads, that is why we had a race. But all threads in the same thread > >group have the same ->sighand, and thus the same ->sighand->siglock. > > > > Oleg, if one thread can be in collect_signal() and another in > sigqueue_free() and both operate on the exact same sigqueue object, its > not clear how we prevent two calls to __sigqueue_free() to > the same object. In that case the lock (or some lock) should be around > __sigqueue_free() - no ? > > i.e if we enter sigqueue_free(), we will call __sigqueue_free() > regardless of the state. Yes. They both will call __sigqueue_free(). But please note that __sigqueue_free() checks SIGQUEUE_PREALLOC, which is cleared by sigqueue_free(). IOW, when sigqueue_free() unlocks ->siglock, we know that it can't be used by collect_signal() from another thread. So we can clear SIGQUEUE_PREALLOC and free sigqueue. We don't need this lock around sigqueue_free() to prevent the race. collect_signal() can "see" only those sigqueues which are on list. IOW, when sigqueue_free() takes ->siglock, colect_signal() can't run, because it needs the same lock. Now we delete this sigqueue from list, nobody can see it, it can't have other references. So we can unlock ->siglock, mark sigqueue as freeable (clear SIGQUEUE_PREALLOC), and free it. Do you agree? Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/