From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757516AbZBSOuN (ORCPT ); Thu, 19 Feb 2009 09:50:13 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752790AbZBSOt7 (ORCPT ); Thu, 19 Feb 2009 09:49:59 -0500 Received: from mx2.redhat.com ([66.187.237.31]:40144 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752784AbZBSOt7 (ORCPT ); Thu, 19 Feb 2009 09:49:59 -0500 Subject: Re: [PATCH] drm: Take mmap_sem up front to avoid lock order violations. From: Kristian =?ISO-8859-1?Q?H=F8gsberg?= To: Peter Zijlstra Cc: krh@bitplanet.net, eric@anholt.net, Wang Chen , dri-devel@lists.sf.net, linux-kernel@vger.kernel.org, Nick Piggin , Andrew Morton , Hugh Dickins In-Reply-To: <1235039601.4612.45.camel@laptop> References: <499BC08C.5000603@cn.fujitsu.com> <1234975113-4941-1-git-send-email-krh@bitplanet.net> <1235035145.4612.38.camel@laptop> <1235039601.4612.45.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Thu, 19 Feb 2009 09:49:40 -0500 Message-Id: <1235054980.31384.16.camel@gaara.bos.redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2009-02-19 at 11:33 +0100, Peter Zijlstra wrote: > On Thu, 2009-02-19 at 10:19 +0100, Peter Zijlstra wrote: > > On Wed, 2009-02-18 at 11:38 -0500, krh@bitplanet.net wrote: > > > From: Kristian Høgsberg > > > > > > A number of GEM operations (and legacy drm ones) want to copy data to > > > or from userspace while holding the struct_mutex lock. However, the > > > fault handler calls us with the mmap_sem held and thus enforces the > > > opposite locking order. This patch downs the mmap_sem up front for > > > those operations that access userspace data under the struct_mutex > > > lock to ensure the locking order is consistent. > > > > > > Signed-off-by: Kristian Høgsberg > > > --- > > > > > > Here's a different and simpler attempt to fix the locking order > > > problem. We can just down_read() the mmap_sem pre-emptively up-front, > > > and the locking order is respected. It's simpler than the > > > mutex_trylock() game, avoids introducing a new mutex. > > > > > OK let me try that again -- my initial response was a tad curt :/ No that's fair, I was aware that the patch was probably borderline and got the feedback I was looking for ;) > While I appreciate your efforts in fixing GEM (I too have an interest in > seeing it done), I cannot support your patch. > > Firstly, you're using mmap_sem well outside its problem domain, this is > bad form. Furthermore, holding it for extended durations for no good > reason affects all other users. Yup, agree. > Secondly, mmap_sem is not a recursive lock (very few kernel locks are, > and we generally frown upon recursive locking schemes), this means that > the fault handler still cannot function properly. I understand, but we take it twice only as a read lock, so that should work, right? We prevent the deadlock the lockdep validator warned about and as far as I can see, the patch doesn't introduce a new one. But other than that I agree with the frowning on recursive locking, it's too often used to paper over badly thought out locking. cheers, Kristian