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

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Date:   Wed, 21 Mar 2018 00:50:18 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     David Miller <davem@...emloft.net>
Cc:     yoshfuji@...ux-ipv6.org, edumazet@...gle.com,
        yanhaishuang@...s.chinamobile.com, nikolay@...ulusnetworks.com,
        yotamg@...lanox.com, soheil@...gle.com, avagin@...tuozzo.com,
        nicolas.dichtel@...nd.com, ebiederm@...ssion.com, fw@...len.de,
        roman.kapl@...go.com, netdev@...r.kernel.org,
        xiyou.wangcong@...il.com, dvyukov@...gle.com,
        andreyknvl@...gle.com, lkp@...el.com
Subject: Re: [PATCH net-next v2 2/5] net: Revert "ipv4: fix a deadlock in
 ip_ra_control"

On 20.03.2018 22:25, Kirill Tkhai wrote:
> Hi, David,
> 
> thanks for the review!
> 
> On 20.03.2018 19:23, David Miller wrote:
>> From: Kirill Tkhai <ktkhai@...tuozzo.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@...tuozzo.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.
>  
>> 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?

We also can just change the order of patches, and make [3/5] go before [2/5].
Then, the kernel still remains bisectable. How do you think about this?

Thanks,
Kirill

>> But you are breaking that here by getting us back into a state
>> where IP_RAW_CONTROL setsockopt will take the socket lock and
>> then RTNL.
>>
>> Again, we can't take, or retake, RTNL if we have the socket lock
>> currently.
>>
>> The only valid locking order is socket lock then RTNL.
> 
> Thanks,
> Kirill
>