netdev - Re: [PATCH] dropmon: add ability to detect when hardware dropsrxpackets

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Date:	Fri, 15 May 2009 07:01:41 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Jiri Pirko <jpirko@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eric Dumazet <dada1@...mosbay.com>, netdev@...r.kernel.org,
	davem@...emloft.net
Subject: Re: [PATCH] dropmon: add ability to detect when hardware
	dropsrxpackets

On Fri, May 15, 2009 at 05:49:47AM +0000, Jarek Poplawski wrote:
> On 14-05-2009 19:29, Neil Horman wrote:
> > On Thu, May 14, 2009 at 02:44:08PM +0200, Jiri Pirko wrote:
> >>> +
> >>> +		/*
> >>> +		 * Clean the device list
> >>> +		 */
> >>> +		list_for_each_entry_rcu(new_stat, &hw_stats_list, list) {
> >> 		^^^^^^^^^^^^^^^^^^^^^^^
> >> This is meaningless here. Use list_for_each_entry_rcu only under rcu_read_lock.
> >> Also it would be good to use list_for_each_entry_safe here since you're
> >> modifying the list.
> >>
> > 
> > The definition of list_for_each_entry_rcu specifically says its safe against
> > list-mutation primitives, so its fine.  Although you are correct, in that its
> > safety is dependent on the protection of rcu_read_lock(), so I'll add that in.
> > Thanks for the catch!  New patch attached
> > 
> > Change notes:
> > 1) Add rcu_read_lock/unlock protection around TRACE_OFF event
> > 
> > Neil
> ...
> >  static int set_all_monitor_traces(int state)
> >  {
> >  	int rc = 0;
> > +	struct dm_hw_stat_delta *new_stat = NULL;
> > +
> > +	spin_lock(&trace_state_lock);
> >  
> >  	switch (state) {
> >  	case TRACE_ON:
> >  		rc |= register_trace_kfree_skb(trace_kfree_skb_hit);
> > +		rc |= register_trace_napi_poll(trace_napi_poll_hit);
> >  		break;
> >  	case TRACE_OFF:
> >  		rc |= unregister_trace_kfree_skb(trace_kfree_skb_hit);
> > +		rc |= unregister_trace_napi_poll(trace_napi_poll_hit);
> >  
> >  		tracepoint_synchronize_unregister();
> > +
> > +		/*
> > +		 * Clean the device list
> > +		 */
> > +		rcu_read_lock();
> > +		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);
> > +			}
> > +		}
> > +		rcu_read_unlock();
> 
> IMHO it looks worse now. rcu_read_lock() suggests it's a read side,
> and spin_lock(&trace_state_lock) protects something else.
> 
the read lock is required (according to the comments for the list loop
primitive) to protect against the embedded mutation primitive, so its required.
I understand that its a bit counterintuitive, but intuition takes a backseat to
functionality. :)
Neil

> Jarek P.
> 
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