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

[Mandeep Singh Baines <msb@chromium.org>]

On Mon, Mar 4, 2013 at 12:09 PM, Mandeep Singh Baines <msb@chromium.org> wrote:
> On Mon, Mar 4, 2013 at 7:53 AM, Myklebust, Trond
> <Trond.Myklebust@netapp.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@redhat.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@chromium.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

D'oh.

s/was/was not/

Regards,
Mandeep

> 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@netapp.com
>> www.netapp.com