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

From: Benjamin Thery <[email-masked]>
Date: Thu, 02 Oct 2008 17:23:30 +0200

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).

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

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

Benjamin

> Jarek P.

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