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

[Josh Hunt <johunt@akamai.com>]

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.

Josh