linux-kernel - Re: [NFS] 2.6.23-rc1-mm2

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Date:	Wed, 8 Aug 2007 02:20:42 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Neil Brown <neilb@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Dietrich <Marc.Dietrich@...physik.uni-giessen.de>,
	Johannes Berg <johannes@...solutions.net>,
	nfs@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [NFS] 2.6.23-rc1-mm2

On 08/07, Trond Myklebust wrote:
>
> On Wed, 2007-08-08 at 01:37 +0400, Oleg Nesterov wrote:
> > On 08/07, Trond Myklebust wrote:
> > >
> > > On Fri, 2007-08-03 at 21:21 +0400, Oleg Nesterov wrote:
> > > > On 08/03, Trond Myklebust wrote:
> > > > > I'll have a look at this. I suspect that most if not all of our calls to
> > > > > run_workqueue()/flush_scheduled_work() can now be replaced by more
> > > > > targeted calls to cancel_work_sync() and cancel_delayed_work_sync().
> > > > 
> > > > Yes, please, if possible.
> > > 
> > > All the NFS and SUNRPC cases appear to be trivial. IOW: the only reason
> > > for the flush_workqueue()/flush_scheduled_work() calls was to ensure
> > > that the cancel_work()/cancel_delayed_work() calls preceding them have
> > > completed. Nevertheless I've split the conversion into two patches,
> > > since one touches only the NFS code, whereas the other touches the
> > > SUNRPC client and server code.
> > > 
> > > The two patches have been tested, and appear to work...
> > 
> > Great!
> > 
> > >  void
> > >  nfs4_kill_renewd(struct nfs_client *clp)
> > >  {
> > >  	down_read(&clp->cl_sem);
> > > -	cancel_delayed_work(&clp->cl_renewd);
> > > +	cancel_delayed_work_sync(&clp->cl_renewd);
> > >  	up_read(&clp->cl_sem);
> > > -	flush_scheduled_work();
> > >  }
> > 
> > this looks unsafe to me, the window is very small, but afaics this can
> > deadlock if called when nfs4_renew_state() has already started, but didn't
> > take ->cl_sem yet.
> 
> Not really. We have removed the nfs_client from the public lists, and we
> are guaranteed that there are no more active superblocks attached to it
> so nothing can call the reclaimer routine (which is the only routine
> that takes a write lock on clp->cl_sem).

Thanks for your explanation. Not that I was able to understand, nfs is a
black magic to me :)

But. nfs4_renew_state() checks list_empty(&clp->cl_superblocks) under
clp->cl_sem? So, if it is possible that clp->cl_renewd was scheduled
at the time when nfs4_kill_renewd(), we can deadlock, no? Because
nfs4_renew_state() needs clp->cl_sem to complete, but nfs4_kill_renewd()
holds this sem, and waits for nfs4_renew_state() completion.

> > Btw, unless I missed something, the code without this patch looks incorrect
> > too: cancel_delayed_work() can fail if the timer expired, but the ->cl_renewd
> > didn't run yet. In that case nfs4_renew_state() can run and re-schedule itself
> > after flush_scheduled_work() returns.
> 
> No, that should not be possible. Again, see above: there are no active
> superblocks, so clp->cl_superblocks is empty.

Yes, thanks. I missed "goto out" in nfs4_renew_state().

Oleg.

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