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

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Message-ID: <20090515192353.GG7745@hmsreliant.think-freely.org>
Date:	Fri, 15 May 2009 15:23:53 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	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 08:11:57PM +0200, Eric Dumazet 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.
> 
Racy in what way?  the rcu_read_lock (As I understand it), prevents list
mutation while we're traversing it here, so we're not going to dereference a
bogus pointer while walking it.  If you're worried about changing the value of a
struct member without an exclusive lock, thats safe in this case, as the worst
case scenario is that we miss detecting a hardware frame drop right before we
stop monitoring the device anyway, and I think thats better than the performance
impact of taking a lock here.

> If caller aleady owns a lock, you dont need :
> 
> rcu_read_lock()
> list_for_each_entry_rcu()
> rcu_read_unlock();
> 
Stephen below points out that notification callbacks always hold the rtnl_lock,
which I wasn't aware of, but I'm not sure thats ok for me to rely on here (at
least I'd rather not)

> > +
> > +		spin_lock(&trace_state_lock);
> > +		if (found && (trace_state == TRACE_OFF)) {
> > +			list_del_rcu(&new_stat->list);
> > +			call_rcu(&new_stat->rcu, free_dm_hw_stat);
> > +		}
> > +		spin_unlock(&trace_state_lock);
> > +		break;
> 
> 
> 
> 
> Its not clear that you use trace_state_lock as the lock guarding all this.
> 
How so?

> If this is the case I suggest a plain and straight forward :
> 
> case NETDEV_UNREGISTER:
> 	spin_lock(&trace_state_lock);
> 	if (trace_state == TRACE_OFF) {
> 		list_for_each_entry(new_stat, &hw_stats_list, list) {
> 			if (new_stat->dev == dev) {
> 				new_stat->dev = NULL;
> 				list_del_rcu(&new_stat->list);
> 				call_rcu(&new_stat->rcu, free_dm_hw_stat);
> 				break;
> 			}
> 		}
> 	}
> 	spin_unlock(&trace_state_lock);
> 	break;
> 
I was hoping to avoid holding the lock while I traversed the entire list.  Not a
huge deal I suppose, but I'd like to avoid doing that if I can.

> 
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