Subject: Re: [PATCH] dropmon: add ability to detect when hardware drops rxpackets

From: Neil Horman <[email-masked]>
Date: Wed, 13 May 2009 20:45:48 -0400

On Wed, May 13, 2009 at 11:23:54AM -0700, Paul E. McKenney wrote:
> On Tue, May 12, 2009 at 12:30:44PM -0400, Neil Horman wrote:
> > On Sat, May 09, 2009 at 08:30:35AM +0200, Eric Dumazet wrote:
> > > 
> > > I dont fully understand your patch, but at least have some questions
> > > about rcu stuff.
> > > 
> > 
> > 
> > Ok, so I went back and I think I managed to better understand the RCU interface.
> > New patch attached, works the same way, saving for the gross previous misuse of
> > rcu.
> > 
> > 
> > Patch to add the ability to detect drops in hardware interfaces via dropwatch.
> > Adds a tracepoint to net_rx_action to signal everytime a napi instance is
> > polled.  The dropmon code then periodically checks to see if the rx_frames
> > counter has changed, and if so, adds a drop notification to the netlink
> > protocol, using the reserved all-0's vector to indicate the drop location was in
> > hardware, rather than somewhere in the code.
> 
> One concern shown below.
> 
<snip>
>  		tracepoint_synchronize_unregister();
> > +
> > +		/*
> > +		 * Clean the device list
> > +		 */
> > +		list_for_each_entry_rcu(new_stat, &hw_stats_list, list) {
> > +			if (new_stat->dev == NULL) {
> > +				list_del_rcu(&new_stat->list);
> > +				call_rcu(&new_stat->rcu, free_dm_hw_stat);
> 
> Much better!  ;-)
> 
Thanks! :)
<snip>

> > +
> > +	switch (event) {
> > +	case NETDEV_REGISTER:
> > +		new_stat = kzalloc(sizeof(struct dm_hw_stat_delta), GFP_KERNEL);
> > +
> > +		if (!new_stat)
> > +			goto out;
> > +
> > +		new_stat->dev = dev;
> > +		INIT_RCU_HEAD(&new_stat->rcu);
> > +		list_add_rcu(&new_stat->list, &hw_stats_list);
> 
> Don't we need to be holding trace_state_lock at this point?  Otherwise,
> can't we mangle the list with a concurrent list_add_rcu() and
> list_del_rcu()?
> 
I thought the purpose of list_add_rcu and list_del_rcu was to be able to modify
lists without needing to hold additional locks.  Or am I missing something else
about the nuance of how RCU works?

Neil