linux-kernel - Re: [PATCH]lockd: fix handling of grace period after long periods of inactivity

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Date:	Tue, 19 Aug 2008 18:12:09 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
Cc:	NAKANO Hiroaki <nakano.hiroaki@....ntt.co.jp>,
	Trond.Myklebust@...app.com, neilb@...e.de,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH]lockd: fix handling of grace period after long periods
	of inactivity

On Fri, Aug 15, 2008 at 10:32:25AM +0900, Fernando Luis Vázquez Cao wrote:
> Hi Bruce!
> 
> On Thu, 2008-08-14 at 15:06 -0400, J. Bruce Fields wrote:
> > On Thu, Aug 14, 2008 at 08:08:16PM +0900, NAKANO Hiroaki wrote:
> > > lockd uses time_before() to determine whether the grace period has
> > > expired. This would seem to be enough to avoid timer wrap-around issues,
> > > but, unfortunately, that is not the case. The time_* family of
> > > comparison functions can be safely used to compare jiffies relatively
> > > close in time, but they stop working after approximately LONG_MAX/2
> > > ticks. nfsd can suffer this problem because the time_before() comparison
> > > in lockd() is not performed until the first request comes in, which
> > > means that if there is no lockd traffic for more than LONG_MAX/2 ticks
> > > we are screwed.
> > > 
> > > The implication of this is that once time_before() starts misbehaving
> > > any attempt from a NFS client to execute fcntl() will be received with a
> > > NLM_LCK_DENIED_GRACE_PERIOD message for 25 days (assuming HZ=1000). In
> > > other words, the 50 seconds grace period could turn into a grace period
> > > of 50 days or more.
> > > 
> > > This patch corrects this behavior by implementing grace period with a
> > > (retriggerable) timer.
> > > 
> > > Note: This bug was analyzed independently by Oda-san <oda@...inux.co.jp>
> > > and myself.
> > 
> > Good catch!  Did you actually run across this in practice?  I would've
> > thought it relatively unusual to have a lockd that didn't receive its
> > first lock request until 25 days after startup.
> Yes, we did find this problem in production. More often than one would
> wish, installing new software in a system that has been running without
> a hiccup for weeks or months is the only thing you will need to bring
> mayhem.
> 
> > I still have a mild preference for a work struct just in case we end up
> > wanting to do something slightly more complicated to end the grace
> > period, but I don't really have anything in mind.
> For simplicity I think we could we get Nakano-san's patch merged first.
> If needed, moving to a work-based solution should be relatively easily.

There's no real difference; patches I've been planning on submitting for
2.6.28 follow.

(We could submit a patch for 2.6.27, since it's a bugfix, but this isn't
a new regression, so existing users at least won't be made any worse
off, and this doesn't crash the server, or anything similarly drastic.
It's still serious, just not quite serious enough to submit at this
point in the release cycle, as I understand the rules....)

--b.
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