From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755428Ab0CISKo (ORCPT ); Tue, 9 Mar 2010 13:10:44 -0500 Received: from www.tglx.de ([62.245.132.106]:45825 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754899Ab0CISKl (ORCPT ); Tue, 9 Mar 2010 13:10:41 -0500 Date: Tue, 9 Mar 2010 19:10:26 +0100 (CET) From: Thomas Gleixner To: Valdis.Kletnieks@vt.edu cc: Lars-Peter Clausen , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [RFC][PATCH] IRQ: Fix oneshot irq race between irq_finalize_oneshot and handle_level_irq In-Reply-To: <5309.1268153945@localhost> Message-ID: References: <1268092679-18070-1-git-send-email-lars@metafoo.de> <5309.1268153945@localhost> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 9 Mar 2010, Valdis.Kletnieks@vt.edu wrote: > On Tue, 09 Mar 2010 08:58:11 +0100, Thomas Gleixner said: > > On Tue, 9 Mar 2010, Lars-Peter Clausen wrote: > > > > > If the kernel has been compiled with preemtion support and handle_level_irq is > > > called from process context for a oneshot irq there is a race between > > > irq_finalize_oneshot and handle_level_irq which results in the irq not being > > > unmasked after its handlers have been run. > > > > > > irq_finalize_oneshot is expected to unmask the irq after the threaded irq > > > handler has been run. It only does so if IRQ_MASKED is set for the irqs status. > > > IRQ_MASKED gets set in the lower part of handle_level_irq after handle_IRQ_event > > > has been called. > > > handle_IRQ_event will wakeup the oneshot irqs threaded handler and if the > > > kernel has been build with preemption there is a chance that the threaded irq > > > handler will finish before execution is returned to handle_level_irq. > > > As a result irq_finalize_oneshot will not unmask the irq and handle_level_irq > > > will set the IRQ_MASKED flag. Thus the irq will stay masked and stalls. > > > > > > In case of an race the call-graph would look like this: > > > handle_level_irq > > > |- mask_ack_irq > > > |- handle_IRQ_event > > > |- wake_up_process > > > |- irq_thread > > > |- action->thread_fn > > > |- irq_finalize_oneshot # Does not unmask the irq > > > |- # Set IRQ_MASKED status flag > > > > Errm, a thread _CANNOT_ preempt a hard interrupt handler. > > What stops the thread from concurrently running on another CPU and racing > that way? I'm an idiot, use small words. :) Right it's a valid SMP problem, but I got confused by the lengthy explanation of a thread preempting an hard interrupt handler. :) Will have a look after dinner. Thanks, tglx