Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751229Ab0L2Jv5 (ORCPT ); Wed, 29 Dec 2010 04:51:57 -0500 Received: from mail-iw0-f194.google.com ([209.85.214.194]:50924 "EHLO mail-iw0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750758Ab0L2Jvz (ORCPT ); Wed, 29 Dec 2010 04:51:55 -0500 MIME-Version: 1.0 X-Originating-IP: [93.172.188.89] In-Reply-To: References: <1292870624-25450-1-git-send-email-felipe.contreras@nokia.com> <1293452486-notmuch-felipe.contreras@nokia.com> From: Ohad Ben-Cohen Date: Wed, 29 Dec 2010 11:51:34 +0200 Message-ID: Subject: Re: [PATCH v2] staging: tidspbridge: protect dmm_map properly To: Felipe Contreras Cc: Felipe Contreras , linux-kernel@vger.kernel.org, linux-omap@vger.kernel.org, greg@kroah.com, omar.ramirez@ti.com, fernando.lugo@ti.com, nm@ti.com, ameya.palande@nokia.com, h-kanigeri2@ti.com Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2135 Lines: 58 On Wed, Dec 29, 2010 at 11:42 AM, Felipe Contreras wrote: > On Tue, Dec 28, 2010 at 4:48 PM, Ohad Ben-Cohen wrote: >> On Tue, Dec 28, 2010 at 2:12 PM, Felipe Contreras >> wrote: >>> On Tue, Dec 28, 2010 at 12:56 PM, Ohad Ben-Cohen wrote: >>>> I still don't know how exactly you triggered the bug: is gst-dsp >>>> multithreaded ? and one of its threads invoked proc_un_map() while >>>> another thread called proc_begin_dma() ? >>> >>> I haven't investigated why that happens >> >> Btw, I still think you should look into this. >> >> The kernel panic will be solved, but you may still have a race there >> that can lead to data corruption: if proc_un_map will be fast enough, >> it will acquire the proc_lock mutex before proc_begin_dma(), and then >> you will miss a cache operation. > > Aquiring the lock is the first thing done; if proc_un_map() aquires > the lock first, it's because it was run first Not true. Again, we have two threads: T1 - called proc_begin_dma() T2 - called proc_un_map() Let's say T1 called proc_begin_dma() first, but it still didn't acquire the lock. At this point the scheduler let T2 run. It calls proc_un_map() and acquire the lock, and free all map_obj's. Then the scheduler let T1 continue to execute, but it can't find any map_obj, and the required cache operation is not performed. Of course the kernel has nothing to do with this, and don't care really. That's why I said it's a user space race - you need to make sure T2 will not call proc_un_map() before T1 (or any other thread you have) completed executing all DMA operations (that you care about). >, and thus a problem for > user-space. If user-space wants the cache operation, it must run > proc_begin_dma() first, there's nothing kernel-space can do to fix > that. > > -- > Felipe Contreras > -- 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/