Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760278AbZAGSrq (ORCPT ); Wed, 7 Jan 2009 13:47:46 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751732AbZAGSrg (ORCPT ); Wed, 7 Jan 2009 13:47:36 -0500 Received: from mx3.mail.elte.hu ([157.181.1.138]:44660 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751489AbZAGSrf (ORCPT ); Wed, 7 Jan 2009 13:47:35 -0500 Date: Wed, 7 Jan 2009 19:47:21 +0100 From: Ingo Molnar To: Alexey Zaytsev Cc: Linus Torvalds , LKML , Nick Piggin , Peter Zijlstra Subject: Re: linux-next: Tree for December 11 Message-ID: <20090107184721.GA16193@elte.hu> References: <20081211200459.14d18d7a.sfr@canb.auug.org.au> <20090107171730.GA21799@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean 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=3.2.3 -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 Content-Length: 1896 Lines: 51 * Alexey Zaytsev wrote: > And last time I bisected, it pointed to: > > commit 7317d7b87edb41a9135e30be1ec3f7ef817c53dd > Author: Nick Piggin > Date: Tue Sep 30 20:50:27 2008 +1000 > > sched: improve preempt debugging > > > This patch helped me out with a problem I recently had.... > > Basically, when the kernel lock is held, then preempt_count > underflow does not > get detected until it is released which may be a long time (and arbitrarily, > eg at different points it may be rescheduled). If the bkl is released at > schedule, the resulting output is actually fairly cryptic... > > With any other lock that elevates preempt_count, it is illegal to schedule > under it (which would get found pretty quickly). bkl allows scheduling with > preempt_count elevated, which makes underflows hard to debug. > > Signed-off-by: Ingo Molnar > > so at least a dumb bisection won't do here. ah, sorry for being a slow starter, i missed that bit - merge window attention span troubles ... I think the kernel_locked() check added here is plain buggy against IRQ contexts: we drop the BKL spinlock and reduce current->kernel_depth non-atomically. So kernel_locked() can become detached from the preempt_count(). Nick, can you think of any better way of still saving this debug check, or should we revert it? Although it seems a bit weird how consistently you seem to be able to trigger it - as this seems to be a narrow race. Is there an IRQ storm there perhaps, or something widens things up for Qemu to inject an IRQ right there? Ingo -- 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/