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SubjectRe: [PATCH]lockd: fix handling of grace period after long periods of inactivity
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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@valinux.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.

I've actually had a patch that does roughly the same thing for a while
at:

git://linux-nfs.org/~bfields/linux.git failover

3ff893a7.. "lockd: don't depend on lockd main loop to end grace" but
hadn't submitted it since I didn't see the bug you found. (I had other
reasons I wanted to do this). Difference that I can see off-hand:

- I used schedule_delayed_work instead of a timer.
- I forgot to delete the thing before exiting!

So I think my solution has a bug that yours doesn't. (I don't see what
would stop the module being removed before my work gets scheduled.)

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.

--b.

>
> Signed-off-by: Nakano Hiroaki <nakano.hiroaki@oss.ntt.co.jp>
> ---
>
> diff -Nrup linux-2.6.27-rc3/fs/lockd/svc.c b/fs/lockd/svc.c
> --- linux-2.6.27-rc3/fs/lockd/svc.c 2008-08-14 16:54:24.000000000 +0900
> +++ b/fs/lockd/svc.c 2008-08-14 17:04:15.000000000 +0900
> @@ -53,6 +53,7 @@ static struct task_struct *nlmsvc_task;
> static struct svc_rqst *nlmsvc_rqst;
> int nlmsvc_grace_period;
> unsigned long nlmsvc_timeout;
> +static struct timer_list nlm_period_timer;
>
> /*
> * These can be set at insmod time (useful for NFS as root filesystem),
> @@ -141,6 +142,12 @@ lockd(void *vrqstp)
>
> grace_period_expire = set_grace_period();
>
> + init_timer(&nlm_period_timer);
> + nlm_period_timer.function = clear_grace_period;
> + nlm_period_timer.expires = grace_period_expire;
> +
> + add_timer(&nlm_period_timer);
> +
> /*
> * The main request loop. We don't terminate until the last
> * NFS mount or NFS daemon has gone away.
> @@ -154,6 +161,7 @@ lockd(void *vrqstp)
> if (nlmsvc_ops) {
> nlmsvc_invalidate_all();
> grace_period_expire = set_grace_period();
> + mod_timer(&nlm_period_timer, grace_period_expire);
> }
> continue;
> }
> @@ -164,10 +172,8 @@ lockd(void *vrqstp)
> * (Theoretically, there shouldn't even be blocked locks
> * during grace period).
> */
> - if (!nlmsvc_grace_period) {
> + if (!nlmsvc_grace_period)
> timeout = nlmsvc_retry_blocked();
> - } else if (time_before(grace_period_expire, jiffies))
> - clear_grace_period();
>
> /*
> * Find a socket with data available and call its
> @@ -195,6 +201,7 @@ lockd(void *vrqstp)
> svc_process(rqstp);
> }
> flush_signals(current);
> + del_timer(&nlm_period_timer);
> if (nlmsvc_ops)
> nlmsvc_invalidate_all();
> nlm_shutdown_hosts();
>
> --
> (NAKANO, Hiroaki) <nakano.hiroaki@oss.ntt.co.jp>
>
> --
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