From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762495AbZEANcg (ORCPT ); Fri, 1 May 2009 09:32:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763344AbZEANbu (ORCPT ); Fri, 1 May 2009 09:31:50 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:60533 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763297AbZEANbs (ORCPT ); Fri, 1 May 2009 09:31:48 -0400 Date: Fri, 1 May 2009 15:31:30 +0200 From: Ingo Molnar To: Chris Mason Cc: Frederic Weisbecker , LKML , Jeff Mahoney , ReiserFS Development List , Alexander Beregalov , Alessio Igor Bogani , Jonathan Corbet , Alexander Viro Subject: Re: [PATCH 4/6] kill-the-BKL/reiserfs: release the write lock inside get_neighbors() Message-ID: <20090501133130.GA31856@elte.hu> References: <1241145862-21700-1-git-send-email-fweisbec@gmail.com> <1241145862-21700-5-git-send-email-fweisbec@gmail.com> <20090501055135.GF5983@elte.hu> <20090501132506.GD6011@nowhere> <1241184543.13084.13.camel@think.oraclecorp.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1241184543.13084.13.camel@think.oraclecorp.com> 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 * Chris Mason wrote: > On Fri, 2009-05-01 at 15:25 +0200, Frederic Weisbecker wrote: > > On Fri, May 01, 2009 at 07:51:35AM +0200, Ingo Molnar wrote: > > > > > > * Frederic Weisbecker wrote: > > > > > > > get_neighbors() is used to get the left and/or right blocks > > > > against a given one in order to balance a tree. > > > > > > > > sb_bread() is used to read the buffer of these neighors blocks and > > > > while it waits for this operation, it might sleep. > > > > > > > > The bkl was released at this point, and then we can also release > > > > the write lock before calling sb_bread(). > > > > > > > > This is safe because if the filesystem is changed after this lock > > > > release, the function returns REPEAT_SEARCH (aka SCHEDULE_OCCURRED > > > > in the function header comments) in order to repeat the neighbhor > > > > research. > > > > > > > > [ Impact: release the reiserfs write lock when it is not needed ] > > > > > > This should also be safe because under the BKL we _already_ dropped > > > the lock when sb_bread() blocked (which it really would in the > > > normal case). > > > > > > There's one special case to consider though: sb_read() maps to > > > __bread() which can return without sleeping if the bh is already > > > uptodate. So if the filesystem _knows_ that the bh is already > > > uptodate and holds a reference to it (this is common pattern in > > > filesystems), it can have a locking assumption on that. > > > > > sb_bread calls __bread which calls __getblk which always calls > might_sleep() before returning. So, the unlock isn't adding a > schedule that wasn't there before. ah, and might_sleep() can schedule under CONFIG_PREEMPT_VOLUNTARY=y. Good point. Ingo