Re: regression 4.4: deadlock in with cgroup percpu_rwsem

[Heiko Carstens]

On Mon, Jan 18, 2016 at 07:48:16PM +0100, Christian Borntraeger wrote:
> 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.

Did the owner field show to something that still looks like a task_struct?