From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754598AbZIAPzK (ORCPT ); Tue, 1 Sep 2009 11:55:10 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753989AbZIAPzJ (ORCPT ); Tue, 1 Sep 2009 11:55:09 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:57758 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753962AbZIAPzI (ORCPT ); Tue, 1 Sep 2009 11:55:08 -0400 Date: Tue, 1 Sep 2009 17:54:27 +0200 From: Ingo Molnar To: Oleg Nesterov Cc: arjan@infradead.org, jeremy@goop.org, mschmidt@redhat.com, mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org, tj@kernel.org, tglx@linutronix.de, Linus Torvalds , Andrew Morton , linux-tip-commits@vger.kernel.org Subject: Re: [PATCH] kthreads: Fix startup synchronization boot crash Message-ID: <20090901155427.GA18078@elte.hu> References: <20090829182718.10f566b1@leela> <20090901100351.GA3361@elte.hu> <20090901113914.GA23578@elte.hu> <20090901130436.GA22514@redhat.com> <20090901131440.GA29783@elte.hu> <20090901133709.GA24041@redhat.com> <20090901135925.GA9083@elte.hu> <20090901145526.GA31317@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090901145526.GA31317@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Oleg Nesterov wrote: > On 09/01, Ingo Molnar wrote: > > > > * Oleg Nesterov wrote: > > > > > On 09/01, Ingo Molnar wrote: > > > > > > > > * Oleg Nesterov wrote: > > > > > > > > > Yes, this should work. But I _think_ we can make the better fix... > > > > > > > > > > I'll try to make the patch soon. Afaics we don't need > > > > > kthreadd_task_init_done. > > > > > > > > ok. > > > > > > Just in case, the patch is ready. [...] > > > > yes - that's roughly the cleanup i referred to in the commit log. > > > > way too late for -rc8 though - the minimal fix i did _might_ be > > eligible. > > > > agreed? > > Agreed. Then I will sent the patch on top of this change. > > But. May be your minimal patch needs a small tweak ? > > rest_init()->complete(&kthreadd_task_init_done) assumes that exactly > _one_ caller of kthread_create() can race with kernel_thread(kthreadd). > Perhap we need complete_all() ? > > > But I must admit, now I don't understand what happens, > > The modification of that variable is protected by the BKL, but > the _ordering_ of the initial task (which becomes the idle > thread of CPU0) and the init task (which is spawned by the > initial task) is not synchronized. > > So we can occasionally end up init running sooner than > rest_init() > > How? rest_init() can't be preempted and it holds BKL. And > kernel_init() takes BKL before anything else. Confused... it cannot be preempted but it can schedule anywhere - and the BKL will be dropped silently. This is one of the biggest dangers of the BKL - rescheduling _somewhere_ in a huge codepath might change timings and 'breaks up the critical path' - breaking ancient assumptions. Ingo