linux-kernel - Re: [PATCH cgroup/for-3.16-fixes] cpuset: break kernfs active protection in cpuset_write

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Date:	Tue, 1 Jul 2014 16:43:26 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH cgroup/for-3.16-fixes] cpuset: break kernfs active
 protection in cpuset_write_resmask()

On Mon, Jun 30, 2014 at 03:47:32PM -0400, Tejun Heo wrote:
> Hey, Li.
> 
> Can you please test this patch and ack it?
> 
> Thanks.
> ------ 8< ------
> Writing to either "cpuset.cpus" or "cpuset.mems" file flushes
> cpuset_hotplug_work so that cpu or memory hotunplug doesn't end up
> migrating tasks off a cpuset after new resources are added to it.
> 
> As cpuset_hotplug_work calls into cgroup core via
> cgroup_transfer_tasks(), this flushing adds the dependency to cgroup
> core locking from cpuset_write_resmak().  This used to be okay because
> cgroup interface files were protected by a different mutex; however,
> 8353da1f91f1 ("cgroup: remove cgroup_tree_mutex") simplified the
> cgroup core locking and this dependency became a deadlock hazard -
> cgroup file removal performed under cgroup core lock tries to drain
> on-going file operation which is trying to flush cpuset_hotplug_work
> blocked on the same cgroup core lock.
> 
> The locking simplification was done because kernfs added an a lot
> easier way to deal with circular dependencies involving kernfs active
> protection.  Let's use the same strategy in cpuset and break active
> protection in cpuset_write_resmask().  While it isn't the prettiest,
> this is a very rare, likely unique, situation which also goes away on
> the unified hierarchy.
> 
> The commands to trigger the deadlock warning without the patch and the
> lockdep output follow.
...
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Li Zefan <lizefan@...wei.com>

Applied to cgroup/for-3.16-fixes.

Thanks.

-- 
tejun
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