From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: regression 4.4: deadlock in with cgroup percpu_rwsem
Date: Mon, 18 Jan 2016 19:48:16 +0100
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Cc: Tejun Heo <tj@kernel.org>,
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To: Peter Zijlstra <peterz@infradead.org>
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On 01/18/2016 07:32 PM, Peter Zijlstra wrote:
> On Fri, Jan 15, 2016 at 04:13:34PM +0100, Christian Borntraeger wrote:
>>> Yes, the deadlock is gone and the system is still running.
>>> After some time I had the following WARN in the logs, though.
>>> Not sure yet if that is related.
>>>
>>> [25331.763607] DEBUG_LOCKS_WARN_ON(lock->owner != current)
>>> [25331.763630] ------------[ cut here ]------------
>>> [25331.763634] WARNING: at kernel/locking/mutex-debug.c:80
> 
>> I restarted the test with panic_on_warn. Hopefully I can get a dump to check
>> which mutex this was.
> 
> Hard to reproduce warnings like this tend to point towards memory
> corruption. Someone stepped on the mutex value and tickles the sanity
> check.
> 
> With lockdep and debugging enabled the mutex gets quite a bit bigger, so
> it gets more likely to be hit by 'random' corruption.
> 
> The locking in seq_read() seems rather straight forward.

I was able to reproduce. The dump shows a mutex that has an owner field, which
does not exists as a task so this all looks fishy. The good thing is, that I
can reproduce the issue within some hours. (exact same backtrace). Will add some
more debug data to get a handle where we come from.