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

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Date:	Wed, 8 Aug 2007 01:37:49 +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 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.

Can't we avoid taking clp->cl_sem here?

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.

Oleg.

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