Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423329AbWKFDIy (ORCPT ); Sun, 5 Nov 2006 22:08:54 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1423333AbWKFDIy (ORCPT ); Sun, 5 Nov 2006 22:08:54 -0500 Received: from smtp.osdl.org ([65.172.181.4]:11227 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1423329AbWKFDIx (ORCPT ); Sun, 5 Nov 2006 22:08:53 -0500 Date: Sun, 5 Nov 2006 19:08:35 -0800 (PST) From: Linus Torvalds To: Steven Rostedt cc: Oleg Nesterov , Thomas Gleixner , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex_slowlock() In-Reply-To: Message-ID: References: <20061105193457.GA3082@oleg> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1473 Lines: 46 On Sun, 5 Nov 2006, Steven Rostedt wrote: > > This whole situation is very theoretical, but I think this actually can > happen *theoretically*. > > OK, the spin_lock doesn't do any serialization, but the unlock does. But > the problem can happen before the unlock. Because of the loop. > > CPU 1 CPU 2 > > task_rq_lock() > > p->state = TASK_RUNNING; > > > (from bottom of for loop) > set_current_state(TASK_INTERRUPTIBLE); > > for (;;) { (looping) > > if (timeout && !timeout->task) > > > (now CPU implements) > t->task = NULL > > task_rq_unlock(); > > schedule() (with state == TASK_INTERRUPTIBLE) Yeah, that seems a real bug. You _always_ need to actually do the thing that you wait for _before_ you want it up. That's what all the scheduling primitives depend on - you can't wake people up first, and then set the condition variable. So if a rt_mutex depeds on something that is set inside the rq-lock, it needs to get the task rw-lock in order to check it. Linus - 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/