linux-kernel - Re: [PATCH 6/6] NLM: Add reference counting to lockd

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Date:	Thu, 10 Jan 2008 06:58:14 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	Neil Brown <neilb@...e.de>
Cc:	akpm@...ux-foundation.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] NLM: Add reference counting to lockd

On Thu, 10 Jan 2008 14:29:22 +1100
Neil Brown <neilb@...e.de> wrote:

> On Tuesday January 8, jlayton@...hat.com wrote:
> > ...and only have lockd exit when the last reference is dropped.
> > 
> > The problem is this:
> > 
> > When a lock that a client is blocking on comes free, lockd does
> > this in nlmsvc_grant_blocked():
> > 
> >     nlm_async_call(block->b_call, NLMPROC_GRANTED_MSG,
> > &nlmsvc_grant_ops);
> > 
> > the callback from this call is nlmsvc_grant_callback(). That
> > function does this at the end to wake up lockd:
> > 
> >     svc_wake_up(block->b_daemon);
> 
> Uhmmm... Maybe there is an easier way.
> 
> block->b_daemon will always be nlmsvc_serv, so can we simply make this
> 
> 	svc_wake_up(nlmsvc_serv);
> with a little locking to make sure nlmsvc_serv is valid?
> 

That's very close to my original patch to fix this problem. I just
replaced svc_wake_up with a call to a new function that wakes up any
lockd that happens to be up. I'm not sure that my original patch was
careful enough with the locking though...

> Actually svc_wake_up is only called from lockd and goes through
> various hoops to find the right rqstp, which we could have known in
> advance.
> So store the rqstp in some global wrapped in a spinlock so we can
> access it safely and just:
> 
>    spin_lock(whatever)
>    if (nlmsvc_rqstp)
> 	wake_up(&nlmsvc_rqstp->rq_wait)
>    spin_unlock(whatever)
> 
> 
> That seems a somewhat simpler way of avoiding the particular problem.
> 

Yes. Much.

> 
> Hmmm.... I guess that nlmsvc_grant_callback could then be run after
> the 'lockd' module had been unloaded.
> Maybe nlm_shutdown_hosts could call rpc_killall_tasks(host->h_rpcclnt)
> on each host.  That should ensure the callback wont happen afterwards.
> 
> Maybe?
> 

I think so. If we let lockd go down before all the RPC's are done,
then the whole problem of accessing lockd data from them sounds like it
could be a problem. If not now, then future changes could cause it.

IIRC, The reason we don't get nlm_destroy_host done on each nlm_host in
this situation is because the h_count is too high. Doing
rpc_killall_tasks in this situation might fix that, but the logic in
all of this is pretty convoluted. I'll see if I can cook up a new
patchset that does this instead.

-- 
Jeff Layton <jlayton@...hat.com>
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