netdev - Re: net: BUG still has locks held in unix_stream_splice

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Date:   Thu, 17 Nov 2016 23:27:17 +0100
From:   Hannes Frederic Sowa <hannes@...essinduktion.org>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Colin Cross <ccross@...roid.com>,
        Mandeep Singh Baines <msb@...omium.org>
Subject: Re: net: BUG still has locks held in unix_stream_splice_read

On 17.11.2016 22:44, Cong Wang wrote:
> On Sun, Oct 9, 2016 at 8:14 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> E.g what will happen if some code does a read on AF_UNIX socket with
>> some local mutex held?  AFAICS, there are exactly two callers of
>> freezable_schedule_timeout() - this one and one in XFS; the latter is
>> in a kernel thread where we do have good warranties about the locking
>> environment, but here it's in the bleeding ->recvmsg/->splice_read and
>> for those assumption that caller doesn't hold any locks is pretty
>> strong, especially since it's not documented anywhere.
>>
>> What's going on there?
> 
> Commit 2b15af6f95 ("af_unix: use freezable blocking calls in read")
> converts schedule_timeout() to its freezable version, it was probably correct
> at that time, but later, commit 2b514574f7e88c8498027ee366
> ("net: af_unix: implement splice for stream af_unix sockets") breaks its
> requirement for a freezable sleep:
> 
>     commit 0f9548ca10916dec166eaf74c816bded7d8e611d
> 
>     lockdep: check that no locks held at freeze time
> 
>     We shouldn't try_to_freeze if locks are held.  Holding a lock can cause a
>     deadlock if the lock is later acquired in the suspend or hibernate path
>     (e.g.  by dpm).  Holding a lock can also cause a deadlock in the case of
>     cgroup_freezer if a lock is held inside a frozen cgroup that is later
>     acquired by a process outside that group.
> 
> So probably we just need to revert commit 2b15af6f95 now.
> 
> I am going to send a revert for at least -net and -stable, since Dmitry
> saw this warning again.

I am not an expert on freezing but this looks around right from the
freezer code. Awesome, thanks a lot for spotting this one!