From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754404Ab3EUMxH (ORCPT ); Tue, 21 May 2013 08:53:07 -0400 Received: from forward-corp1e.mail.yandex.net ([77.88.60.199]:40000 "EHLO forward-corp1e.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754367Ab3EUMxD (ORCPT ); Tue, 21 May 2013 08:53:03 -0400 X-Greylist: delayed 362 seconds by postgrey-1.27 at vger.kernel.org; Tue, 21 May 2013 08:53:03 EDT Authentication-Results: smtpcorp4.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Message-ID: <519B6CBE.1020002@yandex-team.ru> Date: Tue, 21 May 2013 16:46:54 +0400 From: Roman Gushchin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130510 Thunderbird/17.0.6 MIME-Version: 1.0 To: paulmck@linux.vnet.ibm.com CC: Dipankar Sarma , zhmurov@yandex-team.ru, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy Subject: Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro References: <519B38EC.90401@yandex-team.ru> <20130521120906.GD3578@linux.vnet.ibm.com> In-Reply-To: <20130521120906.GD3578@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21.05.2013 16:09, Paul E. McKenney wrote: > On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote: >> Hi, all! >> >> This is a fix for a problem described here: >> https://lkml.org/lkml/2013/4/16/371 . >> --- >> >> Some network functions (udp4_lib_lookup2(), for instance) use the >> hlist_nulls_for_each_entry_rcu macro in a way that assumes restarting >> of a loop. In this case, it is strictly necessary to reread the head->first >> value from the memory before each scan. >> Without additional hints, gcc caches this value in a register. In this case, >> if a cached node is moved to another chain during the scan, we can loop >> forever getting wrong nulls values and restarting the loop uninterruptedly. >> >> Signed-off-by: Roman Gushchin >> Reported-by: Boris Zhmurov >> --- >> include/linux/rculist_nulls.h | 5 +++-- >> 1 file changed, 3 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h >> index 2ae1371..efd51bf 100644 >> --- a/include/linux/rculist_nulls.h >> +++ b/include/linux/rculist_nulls.h >> @@ -37,8 +37,9 @@ static inline void hlist_nulls_del_init_rcu(struct >> hlist_nulls_node *n) >> } >> } >> >> -#define hlist_nulls_first_rcu(head) \ >> - (*((struct hlist_nulls_node __rcu __force **)&(head)->first)) >> +#define hlist_nulls_first_rcu(head) \ >> + (*((struct hlist_nulls_node __rcu __force **) \ >> + &((volatile typeof(*head) *)head)->first)) > > Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here? It will be nice, but will require to keep the old variant too (for using in hlist_nulls_add_head_rcu() as in rcu_assign_pointer() argument). Do you think, it's better? Regards, Roman