From: Ding Tianhong <dingtianhong@huawei•com>
To: David Miller <davem@davemloft•net>
Cc: <gaofeng@cn•fujitsu.com>, <yoshfuji@linux-ipv6•org>,
<joe@perches•com>, <vfalico@redhat•com>, <netdev@vger•kernel.org>
Subject: Re: [PATCH net] net: neighbour: add neighbour dead check for neigh_timer_handler()
Date: Wed, 4 Dec 2013 14:19:06 +0800 [thread overview]
Message-ID: <529EC95A.5080908@huawei.com> (raw)
In-Reply-To: <20131203.232122.852236751455974887.davem@davemloft.net>
On 2013/12/4 12:21, David Miller wrote:
> From: Ding Tianhong <dingtianhong@huawei•com>
> Date: Wed, 4 Dec 2013 12:04:31 +0800
>
>> The destroying neigh could be trigger by userspace, just like set the ip address which
>> in arp table to the local device ip, some I could not control it, it maybe anytime,
>> but the timer handler is execute by logic, this is normal, so I think the logic
>> is no problem, and the process of destroying neigh may conflict with the timer handler,
>> it is a synchronous problem to make sure the timer should be finished before the
>> reference neigh is freed.
>
> The more I think about this, the more none of the explanations for this bug
> make any sense.
>
> neigh_destroy() _ONLY_ runs when:
>
> if (atomic_dec_and_test(&neigh->refcnt))
>
> triggers in neigh_release().
>
> This means it triggers if, and only if, neigh_refcnt goes to zero.
>
> If the refcnt goes to zero, NO TIMER can be running. If the timer is
> running, then there refcnt must be at least '1'.
Hi David:
Yes, you are right, but when the timer is running and prior to get the neigh->lock, the refcnt
could be dec to 0, you could not stop it by existing mechanism.
the refcnt of neighbour could only be inc by these actions:
1.create neighbour, the refcnt will be set to 1.
2.add timer, the refcnt++.
3.neigh_lookup, if found the neigh, refcnt++.
I can show the whole process of my analysis:
CPU 0 CPU 1
----- -----
create_neigh() => refcnt = 1;
add timer => refcnt++;
<SOFTIRQ>
base->running_timer = neigh->timer;
neigh_timer_handler() => at this time, refcnt is 2;
user-> neigh_changeaddr()
neigh_flush_dev();
neigh_del_imer, refcnt dec to 1;
release_neigh(), refcnt is 0,
destroy_neigh()
kfree(neighbour);
write(neigh->lock)
So in my opinion, the point of the problem is that I should not kfree the neighbour until
the timer is not running on CPUs and not pending.
If I miss someghing, pls point out.
Regards
Ding
>
> The only plausible theory would be that something is releasing a neigh
> too early, when references to the neigh still actually exist.
>
> And that's a bug that should be fixed.
>
>
next prev parent reply other threads:[~2013-12-04 6:19 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-03 13:48 [PATCH net] net: neighbour: add neighbour dead check for neigh_timer_handler() Ding Tianhong
2013-12-03 15:03 ` Hannes Frederic Sowa
2013-12-04 1:36 ` Ding Tianhong
2013-12-03 16:28 ` Eric Dumazet
2013-12-04 1:59 ` Ding Tianhong
2013-12-03 16:37 ` David Miller
2013-12-04 2:37 ` Gao feng
2013-12-04 4:04 ` Ding Tianhong
2013-12-04 4:21 ` David Miller
2013-12-04 6:19 ` Ding Tianhong [this message]
2013-12-04 6:27 ` Eric Dumazet
2013-12-04 9:16 ` Ding Tianhong
2013-12-04 10:10 ` Gao feng
2013-12-04 15:24 ` Eric Dumazet
2013-12-05 0:32 ` Gao feng
2013-12-05 3:17 ` Ding Tianhong
2013-12-18 6:37 ` Ding Tianhong
2013-12-18 7:51 ` Hannes Frederic Sowa
2013-12-18 8:19 ` Ding Tianhong
2013-12-18 8:41 ` Hannes Frederic Sowa
2013-12-18 8:57 ` Ding Tianhong
2013-12-18 9:28 ` Hannes Frederic Sowa
2013-12-18 10:02 ` Ding Tianhong
2013-12-18 10:21 ` Hannes Frederic Sowa
2013-12-18 11:57 ` Ding Tianhong
2013-12-18 14:27 ` Hannes Frederic Sowa
2013-12-18 15:12 ` Ding Tianhong
2013-12-18 15:46 ` Hannes Frederic Sowa
2013-12-19 3:32 ` Ding Tianhong
2013-12-04 6:36 ` David Miller
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