From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id 7B07E6B01FC for ; Tue, 6 Apr 2010 15:31:41 -0400 (EDT) From: David Howells Subject: [PATCH] radix_tree_tag_get() is not as safe as the docs make out Date: Tue, 06 Apr 2010 20:31:34 +0100 Message-ID: <20100406193134.26429.78585.stgit@warthog.procyon.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org 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 List-ID: 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, proclaims 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". To this end, remove radix_tree_tag_get() from that list, and comment on its definition that the caller is responsible for preventing concurrent access with set/clear. Furthermore, radix_tree_tag_get() is not safe with respect to radix_tree_delete() either as that also modifies the tags directly. An alternative would be to drop the BUG_ON() from radix_tree_tag_get() and note that it may produce an untrustworthy answer if not so protected. Signed-off-by: David Howells --- include/linux/radix-tree.h | 3 +-- lib/radix-tree.c | 4 ++++ 2 files changed, 5 insertions(+), 2 deletions(-) diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h index c5da749..33daa70 100644 --- a/include/linux/radix-tree.h +++ b/include/linux/radix-tree.h @@ -100,14 +100,13 @@ do { \ * The notable exceptions to this rule are the following functions: * radix_tree_lookup * radix_tree_lookup_slot - * radix_tree_tag_get * radix_tree_gang_lookup * radix_tree_gang_lookup_slot * radix_tree_gang_lookup_tag * radix_tree_gang_lookup_tag_slot * radix_tree_tagged * - * The first 7 functions are able to be called locklessly, using RCU. The + * The first 6 functions are able to be called locklessly, using RCU. The * caller must ensure calls to these functions are made within rcu_read_lock() * regions. Other readers (lock-free or otherwise) and modifications may be * running concurrently. diff --git a/lib/radix-tree.c b/lib/radix-tree.c index 6b9670d..795a3bb 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 + * + * The caller must make sure this function does not run concurrently with + * radix_tree_tag_set/clear() or radix_tree_delete() as these modify the nodes + * directly to alter the tags. */ int radix_tree_tag_get(struct radix_tree_root *root, unsigned long index, unsigned int tag) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org