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Date: Fri, 12 Sep 2008 16:46:52 +0200
From: Eric Dumazet <dada1@...mosbay.com>
To: Benjamin Thery <benjamin.thery@...l.net>
Cc: "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/1] net: fix scheduling of dst_gc_task by __dst_free
Benjamin Thery a écrit :
> The dst garbage collector dst_gc_task() may not be scheduled as we
> expect it to be in __dst_free().
>
> Indeed, when the dst_gc_timer was replaced by the delayed_work
> dst_gc_work, the mod_timer() call used to schedule the garbage
> collector at an earlier date was replaced by a schedule_delayed_work()
> (see commit 86bba269d08f0c545ae76c90b56727f65d62d57f).
>
> But, the behaviour of mod_timer() and schedule_delayed_work() is
> different in the way they handle the delay.
>
> mod_timer() stops the timer and re-arm it with the new given delay,
> whereas schedule_delayed_work() only check if the work is already
> queued in the workqueue (and queue it (with delay) if it is not)
> BUT it does NOT take into account the new delay (even if the new delay
> is earlier in time).
> schedule_delayed_work() returns 0 if it didn't queue the work,
> but we don't check the return code in __dst_free().
>
> If I understand the code in __dst_free() correctly, we want dst_gc_task
> to be queued after DST_GC_INC jiffies if we pass the test (and not in
> some undetermined time in the future), so I think we should add a call
> to cancel_delayed_work() before schedule_delayed_work(). Patch below.
>
Well, you are right that time is undetermined (but < ~120 seconds), so your patch
makes sense.
Acked-by: Eric Dumazet <dada1@...mosbay.com>
Then we should ask why we reset the timer back to its minimum value
every time we call __dst_free(). On machines with many dormant tcp sessions,
dst_garbage.list can contain huge number of non freeable entries :(
Maybe we should count the entries and change the timer only if really needed.
> Or we should at least test the return code of schedule_delayed_work(),
> and reset the values of dst_garbage.timer_inc and dst_garbage.timer_expires
> back to their former values if schedule_delayed_work() failed.
> Otherwise the subsequent calls to __dst_free will test the wrong values
> and assume wrong thing about when the garbage collector is supposed to
> be scheduled.
>
> dst_gc_task() also calls schedule_delayed_work() without checking
> its return code (or calling cancel_scheduled_work() first), but it
> should fine there: dst_gc_task is the routine of the delayed_work, so
> no dst_gc_work should be pending in the queue when it's running.
>
> This patch applies on top of net-2.6.
>
> (Sorry, I think I've been a bit verbose to expose this simple issue :)
>
> Signed-off-by: Benjamin Thery <benjamin.thery@...l.net>
> ---
> net/core/dst.c | 1 +
> 1 file changed, 1 insertion(+)
>
> Index: net-2.6/net/core/dst.c
> ===================================================================
> --- net-2.6.orig/net/core/dst.c
> +++ net-2.6/net/core/dst.c
> @@ -203,6 +203,7 @@ void __dst_free(struct dst_entry * dst)
> if (dst_garbage.timer_inc > DST_GC_INC) {
> dst_garbage.timer_inc = DST_GC_INC;
> dst_garbage.timer_expires = DST_GC_MIN;
> + cancel_delayed_work(&dst_gc_work);
> schedule_delayed_work(&dst_gc_work, dst_garbage.timer_expires);
> }
> spin_unlock_bh(&dst_garbage.lock);
>
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