From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756007Ab3F1Rez (ORCPT ); Fri, 28 Jun 2013 13:34:55 -0400 Received: from mail-qe0-f54.google.com ([209.85.128.54]:46462 "EHLO mail-qe0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755897Ab3F1Rex (ORCPT ); Fri, 28 Jun 2013 13:34:53 -0400 Date: Fri, 28 Jun 2013 10:34:48 -0700 From: Tejun Heo To: "Paul E. McKenney" , Dipankar Sarma Cc: Fengguang Wu , "David S. Miller" , Li Zefan , Patrick McHardy , linux-kernel@vger.kernel.org Subject: [PATCH v3] rculist: list_first_or_null_rcu() should use list_entry_rcu() Message-ID: <20130628173448.GD18889@mtj.dyndns.org> References: <20130621003244.GD11837@mtj.dyndns.org> <20130626172753.GC4405@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130626172753.GC4405@mtj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org list_first_or_null() should test whether the list is empty and return pointer to the first entry if not in a RCU safe manner. It's broken in several ways. * It compares __kernel @__ptr with __rcu @__next triggering the following sparse warning. net/core/dev.c:4331:17: error: incompatible types in comparison expression (different address spaces) * It doesn't perform rcu_dereference*() and computes the entry address using container_of() directly from the __rcu pointer which is inconsitent with other rculist interface. As a result, all three in-kernel users - net/core/dev.c, macvlan, cgroup - are buggy. They dereference the pointer w/o going through read barrier. * While ->next dereference passes through list_next_rcu(), the compiler is still free to fetch ->next more than once and thus nullify the "__ptr != __next" condition check. Fix it by making list_first_or_null_rcu() dereference ->next directly using ACCESS_ONCE() and then use list_entry_rcu() on it like other rculist accessors. v2: Paul pointed out that the compiler may fetch the pointer more than once nullifying the condition check. ACCESS_ONCE() added on ->next dereference. v3: Restored () around macro param which was accidentally removed. Spotted by Paul. Signed-off-by: Tejun Heo Reported-by: Fengguang Wu Cc: Dipankar Sarma Cc: "Paul E. McKenney" Cc: "David S. Miller" Cc: Li Zefan Cc: Patrick McHardy Cc: stable@vger.kernel.org --- include/linux/rculist.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/include/linux/rculist.h b/include/linux/rculist.h index 8089e35..523f13c 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -267,8 +267,9 @@ static inline void list_splice_init_rcu(struct list_head *list, */ #define list_first_or_null_rcu(ptr, type, member) \ ({struct list_head *__ptr = (ptr); \ - struct list_head __rcu *__next = list_next_rcu(__ptr); \ - likely(__ptr != __next) ? container_of(__next, type, member) : NULL; \ + struct list_head *__next = ACCESS_ONCE(__ptr->next); \ + likely(__ptr != __next) ? \ + list_entry_rcu(__next, type, member) : NULL; \ }) /**