Re: Bug in net/ipv6/ip6_fib.c:fib6_dump_table()

[Eric Dumazet <eric.dumazet@gmail.com>]

On Fri, 2012-06-22 at 01:49 -0500, Josh Hunt wrote:
> On 06/21/2012 03:27 PM, Eric Dumazet wrote:
> > On Thu, 2012-06-21 at 14:35 -0500, Josh Hunt wrote:
> > 
> >> Can anyone provide details of the crash which was intended to be fixed
> >> by 2bec5a369ee79576a3eea2c23863325089785a2c? With this patch in and
> >> doing concurrent adds/deletes and dumping the table via netlink causes
> >> duplicate entries to be reported. Reverting this patch causes those
> >> problems to go away. We can provide a more detailed test if that is
> >> needed, but so far our testing has been unable to reproduce the crash
> >> mentioned in the above commit with it reverted.
> > 
> > A mere revert wont be enough.
> > 
> > Looking at this code, it lacks proper synchronization
> > between tree updaters and tree walkers.
> > 
> > fib6_walker_lock rwlock is not enough to prevent races.
> > 
> > Are you willing to fix this yourself ?
> > 
> 
> Looking through the code a bit more it seems like we would need to have
> a lock in fib6_walker_t to protect its contents. Mainly for when we
> update the pointers in fib6_del_route and fib6_repair_tree. Right now
> there is the fib6_walker_lock, but that appears to only be protecting
> the elements of the list, not their contents. Is this what you had in
> mind? I just coded up something along these lines and it works for the
> most part, but I also got a message about unsafe lock ordering when I
> stressed it so I am messing something up. If this sounds like it's on
> the right track I can work out the kinks in the morning.

Hmm, it seems tb6_lock is held by a writer, so its safe :

a tree walker can run only holding a read_lock on tb6_lock