From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757774Ab0DFXfF (ORCPT ); Tue, 6 Apr 2010 19:35:05 -0400 Received: from bld-mail19.adl2.internode.on.net ([150.101.137.104]:47018 "EHLO mail.internode.on.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1757592Ab0DFXe5 (ORCPT ); Tue, 6 Apr 2010 19:34:57 -0400 Date: Wed, 7 Apr 2010 09:34:38 +1000 From: Dave Chinner To: Nick Piggin Cc: David Howells , paulmck@linux.vnet.ibm.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-cachefs@redhat.com Subject: Re: An incorrect assumption over radix_tree_tag_get() Message-ID: <20100406233438.GH11036@dastard> References: <23428.1270570789@redhat.com> <20100406170903.GH5288@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100406170903.GH5288@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 07, 2010 at 03:09:03AM +1000, Nick Piggin wrote: > On Tue, Apr 06, 2010 at 05:19:49PM +0100, David Howells wrote: > > > > Hi, > > > > I think I've made a bad assumption over my usage of radix_tree_tag_get() in > > fs/fscache/page.c. > > > > I've assumed that radix_tree_tag_get() is protected from radix_tree_tag_set() > > and radix_tree_tag_clear() by the RCU read lock. However, now I'm not so > > sure. I think it's only protected against removal of part of the tree. > > > > Can you confirm? > > It is safe. Synchronization requirements for using the radix tree API > are documented. I don't think it is safe - I made modifications to XFS that modified radix tree tags under a read lock (not RCU), but this resulted in corrupted tag state as concurrent tag set/clear operations for different slots propagated through the tree and got mixed up. Christoph fixed the problem (f1f724e4b523d444c5a598d74505aefa3d6844d2) by putting all tag modifications under the write lock. I can't see how doing tag modifications under RCU read locks is any safer than doing it under a spinning read lock.... Cheers, Dave. -- Dave Chinner david@fromorbit.com