netdev - Re: [PATCH] drop_monitor: convert to modular building

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Date:	Wed, 16 May 2012 16:01:45 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	<netdev@...r.kernel.org>, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] drop_monitor: convert to modular building

On Wed, 2012-05-16 at 10:27 -0400, Neil Horman wrote:
> When I first wrote drop monitor I wrote it to just build monolithically.  There
> is no reason it can't be built modularly as well, so lets give it that
> flexibiity.

Yes, please.

[...]
> --- a/net/core/drop_monitor.c
> +++ b/net/core/drop_monitor.c
> @@ -22,6 +22,7 @@
>  #include <linux/timer.h>
>  #include <linux/bitops.h>
>  #include <linux/slab.h>
> +#include <linux/module.h>
>  #include <net/genetlink.h>
>  #include <net/netevent.h>
>  
> @@ -223,9 +224,15 @@ static int set_all_monitor_traces(int state)
>  
>  	switch (state) {
>  	case TRACE_ON:
> +		if (!try_module_get(THIS_MODULE)) {
> +			rc = -EINVAL;

Minor issue, but this isn't the right error code - there is nothing
invalid about the request, it just came at the wrong time.  Perhaps
ENODEV or ECANCELED?

> +			break;
> +		}
> +
>  		rc |= register_trace_kfree_skb(trace_kfree_skb_hit, NULL);
>  		rc |= register_trace_napi_poll(trace_napi_poll_hit, NULL);
>  		break;
> +
>  	case TRACE_OFF:
>  		rc |= unregister_trace_kfree_skb(trace_kfree_skb_hit, NULL);
>  		rc |= unregister_trace_napi_poll(trace_napi_poll_hit, NULL);
> @@ -241,6 +248,9 @@ static int set_all_monitor_traces(int state)
>  				kfree_rcu(new_stat, rcu);
>  			}
>  		}
> +
> +		module_put(THIS_MODULE);
> +
>  		break;
>  	default:
>  		rc = 1;
> @@ -383,4 +393,38 @@ out:
>  	return rc;
>  }
>  
> -late_initcall(init_net_drop_monitor);
> +static void exit_net_drop_monitor(void)
> +{
> +	struct per_cpu_dm_data *data;
> +	int cpu;
> +
> +	if (unregister_netdevice_notifier(&dropmon_net_notifier))
> +		printk(KERN_WARNING "Unable to unregiser dropmon notifer\n");

Currently this will only fail if you didn't actually register the
notifier, which would be a bug.  If there is ever any other reason this
could fail, continuing with the notifier still registered would be
disastrous.  Therefore I think this should be:

	rc = unregister_netdevice_notifier(&dropmon_net_notifier);
	BUG_ON(rc);

> +	/*
> +	 * Because of the module_get/put we do in the trace state change path
> +	 * we are guarnateed not to have any current users when we get here
> +	 * all we need to do is make sure that we don't have any running timers
> +	 * or pending schedule calls
> +	 */

Surely you need to call set_all_monitor_traces(TRACE_OFF) first...

> +	for_each_present_cpu(cpu) {
> +		data = &per_cpu(dm_cpu_data, cpu);
> +		del_timer(&data->send_timer);
> +		cancel_work_sync(&data->dm_alert_work);
> +		/*
> +		 * At this point, we should have exclusive access
> +		 * to this struct and can free the skb inside it
> +		 */
> +		kfree_skb(data->skb);
> +	}
> +
> +	if (genl_unregister_family(&net_drop_monitor_family))
> +		printk(KERN_WARNING "Unable to unregister drop monitor socket family\n");

Same issue as with unregister_netdevice_notifier().

Ben.

> +}
> +
> +module_init(init_net_drop_monitor);
> +module_exit(exit_net_drop_monitor);
> +
> +MODULE_LICENSE("GPL v2");
> +MODULE_AUTHOR("Neil Horman <nhorman@...driver.com>");

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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