linux-kernel - Re: regression 4.4: deadlock in with cgroup percpu

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Date:	Mon, 18 Jan 2016 19:48:16 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org >> Linux Kernel Mailing List" 
	<linux-kernel@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: regression 4.4: deadlock in with cgroup percpu_rwsem

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.