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Date:   Fri, 12 May 2017 12:33:01 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Vovo Yang <vovoy@...gle.com>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: Threads stuck in zap_pid_ns_processes()

Guenter Roeck <linux@...ck-us.net> writes:

> Hi Eric,
>
> On Fri, May 12, 2017 at 08:26:27AM -0500, Eric W. Biederman wrote:
>> Vovo Yang <vovoy@...gle.com> writes:
>> 
>> > On Fri, May 12, 2017 at 7:19 AM, Eric W. Biederman
>> > <ebiederm@...ssion.com> wrote:
>> >> Guenter Roeck <linux@...ck-us.net> writes:
>> >>
>> >>> What I know so far is
>> >>> - We see this condition on a regular basis in the field. Regular is
>> >>>   relative, of course - let's say maybe 1 in a Milion Chromebooks
>> >>>   per day reports a crash because of it. That is not that many,
>> >>>   but it adds up.
>> >>> - We are able to reproduce the problem with a performance benchmark
>> >>>   which opens 100 chrome tabs. While that is a lot, it should not
>> >>>   result in a kernel hang/crash.
>> >>> - Vovo proviced the test code last night. I don't know if this is
>> >>>   exactly what is observed in the benchmark, or how it relates to the
>> >>>   benchmark in the first place, but it is the first time we are actually
>> >>>   able to reliably create a condition where the problem is seen.
>> >>
>> >> Thank you.  I will be interesting to hear what is happening in the
>> >> chrome perfomance benchmark that triggers this.
>> >>
>> > What's happening in the benchmark:
>> > 1. A chrome renderer process was created with CLONE_NEWPID
>> > 2. The process crashed
>> > 3. Chrome breakpad service calls ptrace(PTRACE_ATTACH, ..) to attach to every
>> >   threads of the crashed process to dump info
>> > 4. When breakpad detach the crashed process, the crashed process stuck in
>> >   zap_pid_ns_processes()
>> 
>> Very interesting thank you.
>> 
>> So the question is specifically which interaction is causing this.
>> 
>> In the test case provided it was a sibling task in the pid namespace
>> dying and not being reaped.  Which may be what is happening with
>> breakpad.  So far I have yet to see kernel bug but I won't rule one out.
>> 
>
> I am trying to understand what you are looking for. I would have thought
> that both the test application as well as the Chrome functionality
> described above show that there are situations where zap_pid_ns_processes()
> can get stuck and cause hung task timeouts in conjunction with the use of
> ptrace().
>
> Your last sentence seems to suggest that you believe that the kernel might
> do what it is expected to do. Assuming this is the case, what else would
> you like to see ? A test application which matches exactly the Chrome use
> case ? We can try to provide that, but I don't entirely understand how
> that would change the situation. After all, we already know that it is
> possible to get a thread into this condition, and we already have one
> means to reproduce it.
>
> Replacing TASK_UNINTERRUPTIBLE with TASK_INTERRUPTABLE works for both the
> test application and the Chrome benchmark. The thread is still stuck in
> zap_pid_ns_processes(), but it is now in S (sleep) state instead of D,
> and no longer results in a hung task timeout. It remains in that state
> until the parent process terminates. I am not entirely happy with it
> since the processes are still stuck and may pile up over time, but at
> least it solves the immediate problem for us.
>
> Question now is what to do with that solution. We can of course apply
> it locally to Chrome OS, but I would rather have it upstream - especially
> since we have to assume that any users of Chrome on Linux, or more
> generically anyone using ptrace in conjunction with CLONE_NEWPID, may
> experience the same problem. Right now I have no idea how to get there,
> though. Can you provide some guidance ?

Apologies for not being clear.  I intend to send a pull request with the
the TASK_UINTERRUPTIBLE to TASK_INTERRUPTIBLE change to Linus in the
next week or so with a Cc stable and an appropriate Fixes tag.  So the
fix can be backported.

I have a more comprehensive change queued I will probably merge for 4.13
already but it just changes what kind of zombies you see.  It won't
remove the ``stuck'' zombies.

So what I am looking for now is:
Why are things getting stuck in your benchmark?

-  Is it a userspace bug?

  In which case we can figure out what userspace (aka breakpad) needs
   to do to avoid the problem.
   
-  Is it a kernel bug with ptrace?

   There have been a lot of little subtle bugs with ptrace over the
   years so one more would not surprise

So I am just looking to make certain we fix the root issue not just
the hung task timeout warning.

Eric

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