Subject: Re: [PATCH] net: deadlock during net device unregistration - V2

From: Jarek Poplawski <[email-masked]>
Date: Thu, 2 Oct 2008 20:38:19 +0200

On Thu, Oct 02, 2008 at 05:23:30PM +0200, Benjamin Thery wrote:
> Jarek Poplawski wrote:
>> On Wed, Oct 01, 2008 at 11:06:22PM +0200, Daniel Lezcano wrote:
>>> Jarek Poplawski wrote:
>>>> On Wed, Oct 01, 2008 at 04:14:35PM +0200, Benjamin Thery wrote:
>> ...
>>>>> --- net-next-2.6.orig/net/core/dst.c
>>>>> +++ net-next-2.6/net/core/dst.c
>>>>> @@ -328,6 +328,10 @@ static int dst_dev_event(struct notifier
>>>>>  			dst_ifdown(dst, dev, event != NETDEV_DOWN);
>>>>>  		}
>>>>>  		mutex_unlock(&dst_gc_mutex);
>>>>> +
>>>>> +		if (event == NETDEV_UNREGISTER &&
>>>>> +		    cancel_delayed_work(&dst_gc_work))
>>>>> +			dst_gc_task(&dst_gc_work.work);
>>>> Hmm... It seems this shouldn't work yet: cancel_delayed_work() can only
>>>> kill this while on timer, but not when queued and maybe blocked already.
>
> You're right.
>
>>
>> Hmm#2... Then maybe something like this?:
>>
>> 		if (event == NETDEV_UNREGISTER &&
>> 	    	    (cancel_delayed_work(&dst_gc_work) ||
>> 		     delayed_work_pending(&dst_gc_work)))
>> 			dst_gc_task(&dst_gc_work.work);
>
> Hmmm... I'm not sure I understand what this change do?
>
> OK, I see this ensure we will run dst_gc_task() even if
> cancel_delayed_work() failed and the work is still pending (ie. the  
> timer has expired and dst_gc_work is already in the queue).

I think, this covers exactly the case of blocking you described, plus
more... (the work is queued but not blocked). 

>
> But what if the work was not pending at all at beginning?
> We still need to run dst_gc_task().

Maybe I miss something, but if this is really needed here then it
looks like we are fixing more than "the blocking" bug BTW.

>
> Is something like this better?
> (code expanded to be more readable)
>
> 	if (event == NETDEV_UNREGISTER) {
> 	    if (!delayed_work_pending(&dst_gc_work))
> 	        /* work is not scheduled (no timer, not in queue) */

		/* may be running too */

> 		dst_gc_task(&dst_gc_work.work);
> 	    else if (cancel_delayed_work(&dst_gc_work) ||
>  	             delayed_work_pending(&dst_gc_work)))
>                 /* work was scheduled (and may be blocked) */

		/* actually could be both running and pending here:
		 * if it's after rearming
		 */

>  	        dst_gc_task(&dst_gc_work.work);
> 	    else
>                 /* dst_gc_task() is running, do nothing */

So again !delayed_work_pending() - there could be the change of state
while checking - but then looks a bit inconsistent. I think this should
be OK too.

As a matter of fact I've thought about something even simpler, which
probably should help for all above concerns too:

	if (event == NETDEV_UNREGISTER)
  	        dst_gc_task(&dst_gc_work.work);

dst_gc_task() locking allows for this, and running this two times in
a row could be even faster than trying to cancel the unnecessary run.

Jarek P.