Re: [PATCH net-next v2 2/5] net: Revert "ipv4: fix a deadlock in ip_ra_control"

[David Miller <davem@davemloft.net>]

From: Kirill Tkhai <ktkhai@virtuozzo.com>
Date: Tue, 20 Mar 2018 22:25:35 +0300

> On 20.03.2018 19:23, David Miller wrote:
>> From: Kirill Tkhai <ktkhai@virtuozzo.com>
>> Date: Mon, 19 Mar 2018 12:14:54 +0300
>> 
>>> This reverts commit 1215e51edad1.
>>> Since raw_close() is used on every RAW socket destruction,
>>> the changes made by 1215e51edad1 scale sadly. This clearly
>>> seen on endless unshare(CLONE_NEWNET) test, and cleanup_net()
>>> kwork spends a lot of time waiting for rtnl_lock() introduced
>>> by this commit.
>>>
>>> Next patches in series will rework this in another way,
>>> so now we revert 1215e51edad1. Also, it doesn't seen
>>> mrtsock_destruct() takes sk_lock, and the comment to the commit
>>> does not show the actual stack dump. So, there is a question
>>> did we really need in it.
>>>
>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> 
>> Kirill, I think the commit you are reverting is legitimate.
>> 
>> The IP_RAW_CONTROL path has an ABBA deadlock with other paths once
>> you revert this, so you are reintroducing a bug.
> 
> The talk is about IP_ROUTER_ALERT, I assume there is just an erratum.

My bad, I did indeed mean IP_ROUTER_ALERT.

>> All code paths that must take both RTNL and the socket lock must
>> do them in the same order.  And that order is RTNL then socket
>> lock.
> 
> The place I change in this patch is IP_ROUTER_ALERT. There is only
> a call of ip_ra_control(), while this function does not need socket
> lock. Please, see next patch. It moves this ip_ra_control() out
> of socket lock. And it fixes the problem pointed in reverted patch
> in another way. So, if there is ABBA, after next patch it becomes
> solved. Does this mean I have to merge [2/5] and [3/5] together?

Yes, that is what should happen, because the revert by itself
reintroduces the potential ABBA deadlock between the socket lock
and the RTNL mutex.

I'll take a look at the new version of your series.

Thank you.