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

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Date:	Wed, 20 Jan 2016 08:07:40 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christian Borntraeger <borntraeger@...ibm.com>,
	Peter Zijlstra <peterz@...radead.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 Tue, Jan 19, 2016 at 02:38:45PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Tue, Jan 19, 2016 at 08:36:18PM +0100, Christian Borntraeger wrote:
> > No, its not a task_struct. Activating some more debug information did indeed 
> > revealed several other issues (overwritten redzones etc). Unfortunately I 
> > only saw the broken things after the facts, so I do not know which code did that.
> > When I disabled the cgroup controllers in libvirt I was no longer able to trigger
> > the bugs. Still trying to narrow things down.
> 
> Hmmm... that's worrying.  CONFIG_DEBUG_PAGEALLOC sometimes can catch
> these sort of bugs red-handed.  Might worth trying.

Christian, just to avoid that you get surprised like I did:
CONFIG_DEBUG_PAGEALLOC requires in the meantime an additional kernel
parameter "debug_pagealloc=on" to be active.

That change was introduced a year ago, so it was probably only me who
wasn't aware of that change :)