Re: namespace: deadlock in dec_pid_namespaces

From	Dmitry Vyukov <dvyukov@google.com>
Newsgroups	linux.kernel
Subject	Re: namespace: deadlock in dec_pid_namespaces
Date	2017-01-23 10:40 +0100
Message-ID	<t2Jgu-4GV-9@gated-at.bofh.it> (permalink)
References	<t1H74-7EL-19@gated-at.bofh.it> <t1Hqq-7M4-31@gated-at.bofh.it> <t1LX4-2d6-11@gated-at.bofh.it> <t1Qam-4J3-11@gated-at.bofh.it> <t1RSN-5Ms-15@gated-at.bofh.it>
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On Sat, Jan 21, 2017 at 1:28 AM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Nikolay Borisov <n.borisov.lkml@gmail.com> writes:
>
>> On 20.01.2017 20:05, Eric W. Biederman wrote:
>>> Nikolay Borisov <n.borisov.lkml@gmail.com> writes:
>>>
>>>> On 20.01.2017 15:07, Dmitry Vyukov wrote:
>>>>> Hello,
>>>>>
>>>>> I've got the following deadlock report while running syzkaller fuzzer
>>>>> on eec0d3d065bfcdf9cd5f56dd2a36b94d12d32297 of linux-next (on odroid
>>>>> device if it matters):
>>>
>>> I am puzzled I thought we had fixed this with:
>>>   add7c65ca426 ("pid: fix lockdep deadlock warning due to ucount_lock")
>>> But apparently not.  We  just moved it from hardirq to softirq context.  Bah.
>>>
>>> Thank you very much for the report.
>>>
>>> Nikolay can you make your change use spinlock_irq?  And have put_ucounts
>>> do spin_lock_irqsave?  That way we just don't care where we call this.
>>
>> Like the one attached?
>
> Exactly thank you.  Dmitry if you have time to test that patch and
> verify it fixes your issue I would appreciate it.
>
>> I haven't really taken careful look as to whether
>> the function where _irq versions do fiddle with irq state, since this
>> might cause a problem if we unconditionally enable them.
>
> In code paths where we can sleep irqs must come in enabled or it's a
> bug.
>
> spin_lock_irq which unconditionally disables irqs is thus safe on the
> allocation path.
>
> Similary spin_unlock_irq which unconditionally enables irqs is also safe
> on the allocation path.

Yes, it fixes the issue for me:

Tested-by: Dmitry Vyukov <dvyukov@google.com>

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namespace: deadlock in dec_pid_namespaces Dmitry Vyukov <dvyukov@google.com> - 2017-01-20 14:10 +0100
  Re: namespace: deadlock in dec_pid_namespaces Nikolay Borisov <n.borisov.lkml@gmail.com> - 2017-01-20 14:30 +0100
    Re: namespace: deadlock in dec_pid_namespaces ebiederm@xmission.com (Eric W. Biederman) - 2017-01-20 19:20 +0100
      Re: namespace: deadlock in dec_pid_namespaces Nikolay Borisov <n.borisov.lkml@gmail.com> - 2017-01-20 23:50 +0100
        Re: namespace: deadlock in dec_pid_namespaces ebiederm@xmission.com (Eric W. Biederman) - 2017-01-21 01:40 +0100
          Re: namespace: deadlock in dec_pid_namespaces Dmitry Vyukov <dvyukov@google.com> - 2017-01-23 10:40 +0100

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