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

From: Eric Dumazet <[email-masked]>
Date: Fri, 15 May 2009 20:11:57 +0200

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();

> +
> +		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.

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;