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

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Message-ID: <20090515195321.GH7745@hmsreliant.think-freely.org>
Date:	Fri, 15 May 2009 15:53:21 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Eric Dumazet <dada1@...mosbay.com>, Jiri Pirko <jpirko@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 11:23:45AM -0700, Stephen Hemminger wrote:
> On Fri, 15 May 2009 20:11:57 +0200
> Eric Dumazet <dada1@...mosbay.com> wrote:
> 
> > Neil Horman a écrit :
> > > +static int dropmon_net_event(struct notifier_block *ev_block,
> > > +			unsigned long event, void *ptr)
> > > +{
> > > +	struct net_device *dev = ptr;
> > > +	struct dm_hw_stat_delta *new_stat = NULL;
> > > +	int found = 0;
> > > +
> > > +	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);
> > > +		spin_lock(&trace_state_lock);
> > > +		list_add_rcu(&new_stat->list, &hw_stats_list);
> > > +		spin_unlock(&trace_state_lock);
> > > +		break;
> > > +	case NETDEV_UNREGISTER:
> > > +		rcu_read_lock();
> > > +		list_for_each_entry_rcu(new_stat, &hw_stats_list, list) {
> > > +			if (new_stat->dev == dev)
> > > +				new_stat->dev = NULL;
> > > +				found = 1;
> > > +				break;
> > > +		}
> > > +		rcu_read_unlock();
> > 
> > This is racy, unless caller already owns a lock.
> > 
> > If caller aleady owns a lock, you dont need :
> > 
> > rcu_read_lock()
> > list_for_each_entry_rcu()
> > rcu_read_unlock();
> 
> RTNL mutex is always held on notification call backs.
> Actually why is trace_state_lock needed at all? Why not
> just use the RTNL mutex?
> 
Theres also a need for this mutex to protect simeoustaneous state changes to the
tracer from userspace.  I suppose I could use the rtnl lock for that, but I
think this is more readable.

Neil

> 
> 
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