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

[Mandeep Singh Baines]

On Wed, Mar 6, 2013 at 10:37 AM, Myklebust, Trond
<Trond.Myklebust@xxxxxxxxxx> wrote:
> On Wed, 2013-03-06 at 13:23 -0500, Jeff Layton wrote:
>> On Wed, 6 Mar 2013 07:59:01 -0800
>> Mandeep Singh Baines <msb@xxxxxxxxxxxx> wrote:
>> > In general, holding a lock and freezing can cause a deadlock if:
>> >
>> > 1) you froze via the cgroup_freezer subsystem and a task in another
>> > cgroup tried to acquire the same lock
>> > 2) the lock was needed later is suspend/hibernate. For example, if the
>> > lock was needed in dpm_suspend by one of the device callbacks. For
>> > hibernate, you also need to worry about any locks that need to be
>> > acquired in order to write to the swap device.
>> > 3) another freezing task blocked on this lock and held other locks
>> > needed later in suspend. If that task were skipped by the freezer, you
>> > would deadlock
>> >
>> > You will block/prevent suspend if:
>> >
>> > 4) another freezing task blocked on this lock and was unable to freeze
>> >
>> > I think 1) and 4) can happen for the NFS/RPC case. Case 1) requires
>> > cgroup freezer. Case 4) while not causing a deadlock could prevent
>> > your laptop/phone from sleeping and end up burning all your battery.
>> > If suspend is initiated via lid close you won't even know about the
>> > failure.
>> >
>>
>> We're aware of #4. That was the intent of adding try_to_freeze() into
>> this codepath in the first place. It's not a great solution for obvious
>> reasons, but we don't have another at the moment.
>>
>> For #1 I'm not sure what to do. I'm that familiar with cgroups or how
>> the freezer works.
>>
>> The bottom line is that we have a choice -- we can either rip out this
>> new lockdep warning, or rip out the code that causes it to fire.
>>
>> If we rip out the warning we may miss some legit cases where we might
>> possibly have hit a deadlock.
>>
>> If we rip out the code that causes it to fire, then we exacerbate the
>> #4 problem above. That will effectively make it so that you can't
>> suspend the host whenever NFS is doing anything moderately active.
>
> #4 is probably the only case where we might want to freeze.
>
> Unless we're in a situation where the network is going down, we can
> usually always make progress with completing the RPC call and finishing
> the system call. So in the case of cgroup_freezer, we only care if the
> freezing cgroup also owns the network device (or net namespace) that NFS
> is using to talk to the server.
>
> As I said, the alternative is to notify NFS that the device is going
> down, and to give it a chance to quiesce itself before that happens.
> This is also the only way to ensure that processes which own locks on
> the server (e.g. posix file locks) have a chance to release them before
> being suspended.
>

So if I'm understanding correctly, the locks are held for bounded time
so under normal situation you don't really need to freeze here. But if
the task that is going to wake you were to freeze then this code path
could block suspend if PF_FREEZER_SKIP were not set.

For this particular case (not in general), instead of calling
try_to_freeze(), what if you only cleared PF_FREEZER_SKIP. For case
#1, you wouldn't block suspend if you're stuck waiting. If you
received the event while processes were freezing, you'd just
try_to_freeze() some place else. Probably on the way back up from the
system call. For case #4, you'd eventually complete the rpc and then
freeze some place else. If you're stuck waiting for the bit because
the process that was going to wake you also got frozen, then the
container would be stuck in the THAWING state. Not catastrophic and
something the administrator could workaround by sequencing things
right.

Does this sound like a reasonable workaround?

Regards,
Mandeep

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