Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754767Ab0LOPmS (ORCPT ); Wed, 15 Dec 2010 10:42:18 -0500 Received: from www.tglx.de ([62.245.132.106]:58111 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752193Ab0LOPmR (ORCPT ); Wed, 15 Dec 2010 10:42:17 -0500 Date: Wed, 15 Dec 2010 16:41:55 +0100 (CET) From: Thomas Gleixner To: Jan Kiszka cc: Jan Kiszka , Avi Kivity , Marcelo Tosatti , "linux-kernel@vger.kernel.org" , kvm , Tom Lyon , Alex Williamson , "Michael S. Tsirkin" Subject: Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state In-Reply-To: <4D08CE44.6000407@siemens.com> Message-ID: References: <4D07F70A.30209@web.de> <4D08CE44.6000407@siemens.com> 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 Content-Length: 1871 Lines: 55 On Wed, 15 Dec 2010, Jan Kiszka wrote: > Am 15.12.2010 14:04, Thomas Gleixner wrote: > > On Wed, 15 Dec 2010, Jan Kiszka wrote: > >> Am 14.12.2010 21:54, Thomas Gleixner wrote: > >>> On Mon, 13 Dec 2010, Jan Kiszka wrote: > >>>> @@ -943,6 +950,9 @@ static struct irqaction *__free_irq(unsigned int irq, void *dev_id) > >>>> /* Make sure it's not being used on another CPU: */ > >>>> synchronize_irq(irq); > >>>> > >>>> + if (single_handler) > >>>> + desc->irq_data.drv_status &= ~IRQS_SHARED; > >>>> + > >>> > >>> What's the reason to clear this flag outside of the desc->lock held > >>> region. > >> > >> We need to synchronize the irq first before clearing the flag. > >> > >> The problematic scenario behind this: An IRQ started in shared mode, > >> this the line was unmasked after the hardirq. Now we clear IRQS_SHARED > >> before calling into the threaded handler. And that handler may now think > >> that the line is still masked as IRQS_SHARED is set. > > > > That should read "not set" I guess. > > Can't remember who wrote this, but that guy might have been too tired > for clear sentences: Yes, of course, we could run into troubles, if > IRQS_SHARED was _not_ set while the IRQ line is unmasked between hard > and threaded handler. > > > Hmm, needs more thoughts :( > > Be warned, might be painful. Talking about headache. Your solution above does not prevent that scenario. CPU 0 CPU 1 synchronize_irq(); hard irq comes in sees shared and unmasks clear IRQS_SHARED thread handler runs and sees !SHARED Same scenario, just moved by a few lines :) Thanks, tglx -- 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/