From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: regression 4.4: deadlock in with cgroup percpu_rwsem Date: Mon, 18 Jan 2016 19:48:16 +0100 Message-ID: <569D3370.6040503@de.ibm.com> References: <56978452.6010606@de.ibm.com> <20160114195630.GA3520@mtj.duckdns.org> <5698A023.9070703@de.ibm.com> <56990C9E.7020801@de.ibm.com> <20160118183205.GW6357@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160118183205.GW6357@twins.programming.kicks-ass.net> Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: Peter Zijlstra Cc: Tejun Heo , "linux-kernel@vger.kernel.org >> Linux Kernel Mailing List" , linux-s390 , KVM list , Oleg Nesterov , "Paul E. McKenney" List-ID: 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.