linux-kernel - Re: LOCKDEP: 3.9-rc1: mount.nfs/4272 still has locks held!

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Date:	Mon, 4 Mar 2013 12:09:26 -0800
From:	Mandeep Singh Baines <msb@...omium.org>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Ming Lei <ming.lei@...onical.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ben Chan <benchan@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: LOCKDEP: 3.9-rc1: mount.nfs/4272 still has locks held!

On Mon, Mar 4, 2013 at 7:53 AM, Myklebust, Trond
<Trond.Myklebust@...app.com> wrote:
> On Mon, 2013-03-04 at 23:33 +0800, Ming Lei wrote:
>> Hi,
>>
>> CC guys who introduced the lockdep change.
>>
>> On Mon, Mar 4, 2013 at 11:04 PM, Jeff Layton <jlayton@...hat.com> wrote:
>>
>> >
>> > I don't get it -- why is it bad to hold a lock across a freeze event?
>>
>> At least this may deadlock another mount.nfs during freezing, :-)
>>
>> See detailed explanation in the commit log:
>>
>> commit 6aa9707099c4b25700940eb3d016f16c4434360d
>> Author: Mandeep Singh Baines <msb@...omium.org>
>> Date:   Wed Feb 27 17:03:18 2013 -0800
>>
>>     lockdep: check that no locks held at freeze time
>>
>>     We shouldn't try_to_freeze if locks are held.  Holding a lock can cause a
>>     deadlock if the lock is later acquired in the suspend or hibernate path
>>     (e.g.  by dpm).  Holding a lock can also cause a deadlock in the case of
>>     cgroup_freezer if a lock is held inside a frozen cgroup that is later
>>     acquired by a process outside that group.
>>
>
> This is bloody ridiculous... If you want to add functionality to
> implement cgroup or per-process freezing, then do it through some other
> api instead of trying to push your problems onto others by adding new
> global locking rules.
>
> Filesystems are a shared resource that have _nothing_ to do with process
> cgroups. They need to be suspended when the network goes down or other
> resources that they depend on are suspended. At that point, there is no
> "what if I launch a new mount command?" scenario.
>

Hi Trond,

My intention was to introduce new rules. My change simply introduces a
check for a deadlock case that can already happen.

I think a deadlock could happen under the following scenario:

1) An administrator wants to freeze a container. Perhaps to checkpoint
it and it migrate it some place else.
2) An nfs mount was in progress so we hit this code path and freeze
with a lock held.
3) Another container tries to nfs mount.
4) Deadlock.

Regards,
Mandeep

> Trond
> --
> Trond Myklebust
> Linux NFS client maintainer
>
> NetApp
> Trond.Myklebust@...app.com
> www.netapp.com
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