Subject: Re: [PATCH 2/5] net: Make rtnetlink infrastructure network namespace aware

From: Patrick McHardy <[email-masked]>
Date: Sun, 30 Sep 2007 17:39:54 +0200

Eric W. Biederman wrote:
> Patrick McHardy <[email-masked]> writes:
> 
> 
>>Maybe I can save you some time: we used to do down_trylock()
>>for the rtnl mutex, so senders would simply return if someone
>>else was already processing the queue *or* the rtnl was locked
>>for some other reason. In the first case the process already
>>processing the queue would also process the new messages, but
>>if it the rtnl was locked for some other reason (for example
>>during module registration) the message would sit in the
>>queue until the next rtnetlink sendmsg call, which is why
>>rtnl_unlock does queue processing. Commit 6756ae4b changed
>>the down_trylock to mutex_lock, so senders will now simply wait
>>until the mutex is released and then call netlink_run_queue
>>themselves. This means its not needed anymore.
> 
> 
> Sounds reasonable.
> 
> I started looking through the code paths and I currently cannot
> see anything that would leave a message on a kernel rtnl socket.
> 
> However I did a quick test adding a WARN_ON if there were any messages
> found in the queue during rtnl_unlock and I found this code path
> getting invoked from linkwatch_event.  So there is clearly something I
> don't understand, and it sounds at odds just a bit from your
> description.

That sounds like a bug. Did you place the WARN_ON before or after
the mutex_unlock()?