Commit 0621ed2e authored Jul 14, 2007 by Patrick McHardy Committed by David S. Miller Jul 14, 2007

[NET_SCHED]: Revert "avoid transmit softirq on watchdog wakeup" optimization



As noticed by Ranko Zivojnovic <ranko@spidernet.net>, calling qdisc_run
from the timer handler can result in deadlock:

> CPU#0
>
> qdisc_watchdog() fires and gets dev->queue_lock
> qdisc_run()...qdisc_restart()...
> -> releases dev->queue_lock and enters dev_hard_start_xmit()
>
> CPU#1
>
> tc del qdisc dev ...
> qdisc_graft()...dev_graft_qdisc()...dev_deactivate()...
> -> grabs dev->queue_lock ...
>
> qdisc_reset()...{cbq,hfsc,htb,netem,tbf}_reset()...qdisc_watchdog_cancel()...
> -> hrtimer_cancel() - waiting for the qdisc_watchdog() to exit, while still
>		        holding dev->queue_lock
>
> CPU#0
>
> dev_hard_start_xmit() returns ...
> -> wants to get dev->queue_lock(!)
>
> DEADLOCK!

The entire optimization is a bit questionable IMO, it moves potentially
large parts of NET_TX_SOFTIRQ work to TIMER_SOFTIRQ/HRTIMER_SOFTIRQ,
which kind of defeats the separation of them.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Ranko Zivojnovic <ranko@spidernet.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 59eecdfb

net/sched/sch_api.c

+1 −5

Original line number	Diff line number	Diff line
		@@ -278,10 +278,6 @@ static enum hrtimer_restart qdisc_watchdog(struct hrtimer *timer)

		wd->qdisc->flags &= ~TCQ_F_THROTTLED;
		smp_wmb();
		if (spin_trylock(&dev->queue_lock)) {
		qdisc_run(dev);
		spin_unlock(&dev->queue_lock);
		} else
		netif_schedule(dev);

		return HRTIMER_NORESTART;