netdev - Re: [Patch net] af_unix: revert "af_unix: use freezable blocking calls in read"

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Date:   Thu, 17 Nov 2016 14:47:10 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Colin Cross <ccross@...roid.com>
Cc:     netdev <netdev@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [Patch net] af_unix: revert "af_unix: use freezable blocking
 calls in read"

On Thu, Nov 17, 2016 at 2:30 PM, Colin Cross <ccross@...roid.com> wrote:
> On Thu, Nov 17, 2016 at 2:09 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> 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 2b514574f7e8
>> ("net: af_unix: implement splice for stream af_unix sockets") breaks
>> the strong requirement for a freezable sleep, according to
>> commit 0f9548ca1091:
>>
>>     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.
>>
>> The pipe_lock is still held at that point. So just revert commit 2b15af6f95.
>
> On my phone 77 threads are blocked in unix_stream_recvmsg.  A simple
> revert of this patch will cause every one of those threads to wake up
> twice per suspend cycle, which can be multiple times a second.  How
> about adding a freezable flag to unix_stream_read_state so
> unix_stream_recvmsg can stay freezable, and unix_stream_splice_read
> can be unfreezable?

Fair enough, I didn't know it could have such an impact.
I will send v2.

Thanks.