Re: [NETPOLL] netconsole: fix soft lockup when removing module

On Sun, Jul 01, 2007 at 09:35:58PM +0400, Oleg Nesterov wrote:

Jarek Poplawski wrote:

quoted

   #1
   Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work()
   required a work function should always (unconditionally) rearm with
   delay > 0 - otherwise it would endlessly loop. This patch replaces
   this function with cancel_delayed_work(). Later kernel versions don't
   require this, so here it's only for uniformity.

But 2.6.22 doesn't need this change, why it was merged?

One bad reason is given above. Should I look for another one?

In fact, I suspect this change adds a race,

You are right!

quoted

--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c

@@ -72,7 +72,8 @@ static void queue_process(struct work_struct *work)
 			netif_tx_unlock(dev);
 			local_irq_restore(flags);
 
-			schedule_delayed_work(&npinfo->tx_work, HZ/10);
+			if (atomic_read(&npinfo->refcnt))
+				schedule_delayed_work(&npinfo->tx_work, HZ/10);
 			return;
 		}
 		netif_tx_unlock(dev);

@@ -785,9 +786,15 @@ void netpoll_cleanup(struct netpoll *np)
 			if (atomic_dec_and_test(&npinfo->refcnt)) {
 				skb_queue_purge(&npinfo->arp_tx);
 				skb_queue_purge(&npinfo->txq);
-				cancel_rearming_delayed_work(&npinfo->tx_work);
+				cancel_delayed_work(&npinfo->tx_work);
 				flush_scheduled_work();

Suppose that ->refcnt == 1, and queue_process() was preempted just after
atomic_read(&npinfo->refcnt).

netpoll_cleanup() comes, cancel_delayed_work() does nothing, flush_scheduled_work()
sleeps.

queue_process() gets CPU, re-schedules ->tx_work, and returns.

flush_scheduled_work() completes, netpoll_cleanup() frees npinfo and returns
while ->tx_work is pending.

No?

No no. (Yes!)

I had some doubts about this, and you found very good reason
for this.

I'll soon send a patch to restore cancel_rearming_delayed_work
in 2.6.22.

So, 2.6.21 needs something better (maybe you've found it btw.?),
but they weren't too interested, anyway.

Thanks very much & regards,
Jarek P.