From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S967917Ab3E3IZp (ORCPT ); Thu, 30 May 2013 04:25:45 -0400 Received: from forward-corp1e.mail.yandex.net ([77.88.60.199]:48287 "EHLO forward-corp1e.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S967723Ab3E3IZf (ORCPT ); Thu, 30 May 2013 04:25:35 -0400 Authentication-Results: smtpcorp4.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Message-ID: <51A70CF4.1020807@yandex-team.ru> Date: Thu, 30 May 2013 12:25:24 +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: Eric Dumazet CC: David Miller , paulmck@linux.vnet.ibm.com, Jesper Dangaard Brouer , Dipankar Sarma , zhmurov@yandex-team.ru, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy , David Laight Subject: Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro References: <519CB2D8.103@yandex-team.ru> <1369225837.3301.324.camel@edumazet-glaptop> <519CC2FB.2010006@yandex-team.ru> <20130522174532.GC3431@linux.vnet.ibm.com> <519D19DA.50400@yandex-team.ru> <20130525113715.GA3795@linux.vnet.ibm.com> <51A39E11.5020405@yandex-team.ru> <1369699930.3301.494.camel@edumazet-glaptop> <51A47496.6000100@yandex-team.ru> <1369787693.3301.586.camel@edumazet-glaptop> <20130529013111.GS6172@linux.vnet.ibm.com> <1369804097.3301.615.camel@edumazet-glaptop> <51A5D3D9.1000106@yandex-team.ru> <1369854387.5109.77.camel@edumazet-glaptop> In-Reply-To: <1369854387.5109.77.camel@edumazet-glaptop> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29.05.2013 23:06, Eric Dumazet wrote: > On Wed, 2013-05-29 at 14:09 +0400, Roman Gushchin wrote: > > True, these lookup functions are usually structured the same around the > hlist_nulls_for_each_entry_rcu() loop. > > A barrier() right before the loop seems to be a benefit, the size of > assembly code is reduced by 48 bytes. > > And its one of the documented way to handle this kind of problems > (Documentation/atomic_ops.txt line 114) > > I guess we should amend this documentation, eventually. > > Thanks, please add you "Signed-off-by" if you agree with the patch. > Signed-off-by: Roman Gushchin Many thanks to you, Paul E. McKenney and David Laight for your patches, help and participation in this discussion. > > [PATCH] net: force a reload of first item in hlist_nulls_for_each_entry_rcu > > Roman Gushchin discovered that udp4_lib_lookup2() was not reloading > first item in the rcu protected list, in case the loop was restarted. > > This produced soft lockups as in https://lkml.org/lkml/2013/4/16/37 > > rcu_dereference(X)/ACCESS_ONCE(X) seem to not work as intended if X is > ptr->field : > > In some cases, gcc caches the value or ptr->field in a register. > > Use a barrier() to disallow such caching, as documented in > Documentation/atomic_ops.txt line 114 > > Thanks a lot to Roman for providing analysis and numerous patches. > > Diagnosed-by: Roman Gushchin > Signed-off-by: Eric Dumazet > Reported-by: Boris Zhmurov > --- > include/linux/rculist_nulls.h | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h > index 2ae1371..c7557fa 100644 > --- a/include/linux/rculist_nulls.h > +++ b/include/linux/rculist_nulls.h > @@ -105,9 +105,14 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n, > * @head: the head for your list. > * @member: the name of the hlist_nulls_node within the struct. > * > + * The barrier() is needed to make sure compiler doesn't cache first element [1], > + * as this loop can be restarted [2] > + * [1] Documentation/atomic_ops.txt around line 114 > + * [2] Documentation/RCU/rculist_nulls.txt around line 146 > */ > #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \ > - for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ > + for (({barrier();}), \ > + pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ > (!is_a_nulls(pos)) && \ > ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \ > pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos))) > >