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

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Message-Id: <20180320.122305.1801176841938756457.davem@davemloft.net>
Date:   Tue, 20 Mar 2018 12:23:05 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     ktkhai@...tuozzo.com
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"

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.

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.

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.

Thank you.