From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757127Ab0DFVhf (ORCPT ); Tue, 6 Apr 2010 17:37:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:14776 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754662Ab0DFVh0 (ORCPT ); Tue, 6 Apr 2010 17:37:26 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells Subject: [PATCH] radix_tree_tag_get() is not as safe as the docs make out [ver #2] To: torvalds@osdl.org, akpm@linux-foundation.org, npiggin@suse.de Cc: paulmck@linux.vnet.ibm.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, David Howells Date: Tue, 06 Apr 2010 22:36:20 +0100 Message-ID: <20100406213620.28251.90764.stgit@warthog.procyon.org.uk> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org radix_tree_tag_get() is not safe to use concurrently with radix_tree_tag_set() or radix_tree_tag_clear(). The problem is that the double tag_get() in radix_tree_tag_get(): if (!tag_get(node, tag, offset)) saw_unset_tag = 1; if (height == 1) { int ret = tag_get(node, tag, offset); may see the value change due to the action of set/clear. RCU is no protection against this as no pointers are being changed, no nodes are being replaced according to a COW protocol - set/clear alter the node directly. The documentation in linux/radix-tree.h, however, says that radix_tree_tag_get() is an exception to the rule that "any function modifying the tree or tags (...) must exclude other modifications, and exclude any functions reading the tree". The problem is that the next statement in radix_tree_tag_get() checks that the tag doesn't vary over time: BUG_ON(ret && saw_unset_tag); This has been seen happening in FS-Cache: https://www.redhat.com/archives/linux-cachefs/2010-April/msg00013.html To this end, remove the BUG_ON() from radix_tree_tag_get() and note in various comments that the value of the tag may change whilst the RCU read lock is held, and thus that the return value of radix_tree_tag_get() may not be relied upon unless radix_tree_tag_set/clear() and radix_tree_delete() are excluded from running concurrently with it. Reported-by: Romain DEGEZ Signed-off-by: David Howells --- include/linux/radix-tree.h | 7 +++++++ lib/radix-tree.c | 12 ++++++------ 2 files changed, 13 insertions(+), 6 deletions(-) diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index c5da749..55ca73c 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -121,6 +121,13 @@ do { \ * (Note, rcu_assign_pointer and rcu_dereference are not needed to control * access to data items when inserting into or looking up from the radix tree) * + * Note that the value returned by radix_tree_tag_get() may not be relied upon + * if only the RCU read lock is held. Functions to set/clear tags and to + * delete nodes running concurrently with it may affect its result such that + * two consecutive reads in the same locked section may return different + * values. If reliability is required, modification functions must also be + * excluded from concurrency. + * * radix_tree_tagged is able to be called without locking or RCU. */ diff --git a/lib/radix-tree.c b/lib/radix-tree.c index 6b9670d..6a75327 100644 --- a/lib/radix-tree.c +++ b/lib/radix-tree.c @@ -556,6 +556,10 @@ EXPORT_SYMBOL(radix_tree_tag_clear); * * 0: tag not present or not set * 1: tag set + * + * Note that the return value of this function may not be relied on, even if + * the RCU lock is held, unless tag modification and node deletion are excluded + * from concurrency. */ int radix_tree_tag_get(struct radix_tree_root *root, unsigned long index, unsigned int tag) @@ -596,12 +600,8 @@ int radix_tree_tag_get(struct radix_tree_root *root, */ if (!tag_get(node, tag, offset)) saw_unset_tag = 1; - if (height == 1) { - int ret = tag_get(node, tag, offset); - - BUG_ON(ret && saw_unset_tag); - return !!ret; - } + if (height == 1) + return !!tag_get(node, tag, offset); node = rcu_dereference_raw(node->slots[offset]); shift -= RADIX_TREE_MAP_SHIFT; height--;