From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932161Ab0DGH5q (ORCPT ); Wed, 7 Apr 2010 03:57:46 -0400 Received: from cantor2.suse.de ([195.135.220.15]:46386 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753020Ab0DGH5i (ORCPT ); Wed, 7 Apr 2010 03:57:38 -0400 Date: Wed, 7 Apr 2010 17:57:30 +1000 From: Nick Piggin To: Dave Chinner 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: <20100407075730.GA5433@laptop> References: <23428.1270570789@redhat.com> <20100406170903.GH5288@laptop> <20100406233438.GH11036@dastard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100406233438.GH11036@dastard> 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 09:34:38AM +1000, Dave Chinner wrote: > 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.... No the modifications must all be serialized, but they can run in parallel with a radix_tree_tag_get().