netdev - Re: Bug in net/ipv6/ip6_fib.c:fib6_dump

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Message-ID: <4FE3E7B0.3020802@cn.fujitsu.com>
Date:	Fri, 22 Jun 2012 11:34:08 +0800
From:	Gao feng <gaofeng@...fujitsu.com>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Josh Hunt <johunt@...mai.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"kaber@...sh.net" <kaber@...sh.net>,
	Debabrata Banerjee <dbavatar@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
	"jmorris@...ei.org" <jmorris@...ei.org>,
	"pekkas@...core.fi" <pekkas@...core.fi>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Bug in net/ipv6/ip6_fib.c:fib6_dump_table()

于 2012年06月22日 05:50, Alexey Kuznetsov 写道:
> On Thu, Jun 21, 2012 at 10:27:49PM +0200, Eric Dumazet wrote:
>> Looking at this code, it lacks proper synchronization
>> between tree updaters and tree walkers.
>>
>> fib6_walker_lock rwlock is not enough to prevent races.
> 
> Hmm. As author of this weird code, I must say I honestly believed it was correct.
> At least I tried. :-)
> 
> 
> What's about 2bec5a336.., it does not look reasonable.
> The idea was that when you change tree, you fixup sleeping walkers, moving
> their location in tree to correct point. So, walkers must not have any stale pointers
> at any times (except when you under table write lock) and no skips/counts are required.
> I remember how damn difficult was it to make this right (well, sorry, if it is not yet :-)),
> so that understand that if some update is forgotten or done incorrectly, it is not so easy to find,
> but it is definitely worth of efforts.


Actually, I spent two months to try to reproduce this crash four months ago,
But finally I give up, I don't think there was any stale pointers,
we already correct it when we change the tree.
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