linux-kernel - Re: [BUG] cgroup/workques/fork: deadlock when moving cgroups

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Message-ID: <20160413194820.GC30260@dhcp22.suse.cz>
Date:	Wed, 13 Apr 2016 21:48:21 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Petr Mladek <pmladek@...e.com>, cgroups@...r.kernel.org,
	Cyril Hrubis <chrubis@...e.cz>, linux-kernel@...r.kernel.org,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [BUG] cgroup/workques/fork: deadlock when moving cgroups

On Wed 13-04-16 15:37:34, Tejun Heo wrote:
> Hello, Michal.
> 
> On Wed, Apr 13, 2016 at 09:23:14PM +0200, Michal Hocko wrote:
> > I think we can live without lru_add_drain_all() in the migration path.
> > We are talking about 4 pagevecs so 56 pages. The charge migration is
> 
> Ah, nice.
> 
> > racy anyway. What concerns me more is how all this is fragile. It sounds
> > just too easy to add a dependency on per-cpu sync work later and
> > reintroduce this issue which is quite hard to detect.
> > Cannot we come up with something more robust?  Or at least warn when we
> > try to use per-cpu workers with problematic locks held?
> 
> Yeah, workqueue does limited lockdep annotation but it doesn't
> integrate fully with the rest of dependency tracking.  It'd be nice to
> have that.  Don't know how to tho.

I was thinking about something like flush_per_cpu_work() which would
assert on group_threadgroup_rwsem held for write. But this still sounds
suboptimal to me. Do we really have to do
cgroup_threadgroup_change_begin for kworker threads? They are
PF_NO_SETAFFINITY so they cannot be moved AFAIR. Or am I missing
something?

-- 
Michal Hocko
SUSE Labs