Subject: Re: [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH

From: Eugene Shatokhin <[email-masked]>
Date: Mon, 31 Aug 2015 11:50:31 +0300

31.08.2015 10:32, Bjørn Mork пишет:
> Eugene Shatokhin <[email-masked]> writes:
>> 28.08.2015 11:55, Bjørn Mork пишет:
>>
>>> I guess you are right.  At least I cannot prove that you are not :)
>>>
>>> There is a bit too much complexity involved here for me...
>>
>> :-)
>>
>> Yes, it is quite complex.
>>
>> I admit, it was easier for me to find the races in usbnet (the tools
>> like KernelStrider and RaceHound do the dirty work) than to analyze
>> their consequences. The latter often requires some time and effort,
>> and so it did this time.
>>
>> Well, any objections to this patch?
>
> No objections from me.
>
> But I would have liked it much better if the code became simpler instead
> of more complex.

Me too, but I can see no other way here. The code is simpler without 
locking, indeed, but locking is needed to prevent the problems described 
earlier.

One needs to make sure that checking if txq or rxq is empty in 
usbnet_terminate_urbs() cannot get inbetween of processing of these 
queues and dev->done in defer_bh(). So 'list' and 'dev->done' must be 
updated under a common lock in defer_bh(). list->lock is an obvious 
candidate for this.

For the same reason, skb_queue_empty(q) must be called under q->lock. So 
the code takes it, calls skb_queue_empty(q) and then releases it to wait 
a little. Rinse and repeat.

The last complex piece is that spin_lock_nested() in defer_bh. It is 
safe to take both list->lock and dev->done.lock there (defer_bh can only 
be called for list = dev->rxq or dev->txq but not for dev->done). For 
lockdep, however, this is suspicious because '*list' and 'dev->done' are 
of the same type so the lock class is the same. So it complained.

To tell lockdep it is OK to use such locking scheme in this particular 
case, the recommended pattern was used: spin_lock_nested with 
SINGLE_DEPTH_NESTING.

Regards,
Eugene