From: Ding Tianhong <dingtianhong@huawei•com>
To: Eric Dumazet <eric.dumazet@gmail•com>
Cc: David Miller <davem@davemloft•net>, <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 17:16:58 +0800 [thread overview]
Message-ID: <529EF30A.4050609@huawei.com> (raw)
In-Reply-To: <1386138457.30495.86.camel@edumazet-glaptop2.roam.corp.google.com>
On 2013/12/4 14:27, Eric Dumazet wrote:
> On Wed, 2013-12-04 at 14:19 +0800, Ding Tianhong wrote:
>> 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;
>
> Nope : del_timer() would return 0 here, so we do not decrement refcnt.
>
The first call for del_timer() will return 1, because the timer->entry.next is not NULL,
then in the neigh_destroy, the del_timer() again will return 0 because timer->entry.next is NULL.
> I can tell you, if this was not the case, a lot of things would be
> terribly broken, like TCP stack.
>
>> 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.
>
> As David explained, if a timer is running, refcnt can not reach 0,
> untill the timer handler finished.
>
> So _something_ is calling neigh_release(n) without prior neigh_hold()
>
Maybe I could try to find more about it, but it is hard to reoccur the problem.
Regards
Ding
>
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe netdev" in
> the body of a message to majordomo@vger•kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
> .
>
next prev parent reply other threads:[~2013-12-04 9:19 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
2013-12-04 6:27 ` Eric Dumazet
2013-12-04 9:16 ` Ding Tianhong [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=529EF30A.4050609@huawei.com \
--to=dingtianhong@huawei$(echo .)com \
--cc=davem@davemloft$(echo .)net \
--cc=eric.dumazet@gmail$(echo .)com \
--cc=gaofeng@cn$(echo .)fujitsu.com \
--cc=joe@perches$(echo .)com \
--cc=netdev@vger$(echo .)kernel.org \
--cc=vfalico@redhat$(echo .)com \
--cc=yoshfuji@linux-ipv6$(echo .)org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox