netdev - Re: [PATCH 06/10] s390: use RCU to walk list of network devices

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Date:	Tue, 10 Nov 2009 10:29:02 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	netdev@...r.kernel.org, linux390@...ibm.com
Subject: Re: [PATCH 06/10] s390: use RCU to walk list of network devices

On Tue, 10 Nov 2009 19:27:11 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > This is similar to other cases where for_each_netdev_rcu
> > can be used when gathering information.
> > 
> > By inspection, don't have platform or cross-build environment
> > to validate.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> > 
> > 
> > --- a/arch/s390/appldata/appldata_net_sum.c	2009-11-09 22:19:05.593480476 -0800
> > +++ b/arch/s390/appldata/appldata_net_sum.c	2009-11-10 09:28:38.335438652 -0800
> > @@ -83,8 +83,9 @@ static void appldata_get_net_sum_data(vo
> >  	rx_dropped = 0;
> >  	tx_dropped = 0;
> >  	collisions = 0;
> > -	read_lock(&dev_base_lock);
> > -	for_each_netdev(&init_net, dev) {
> > +
> > +	rcu_read_lock();
> > +	for_each_netdev_rcu(&init_net, dev) {
> >  		const struct net_device_stats *stats = dev_get_stats(dev);
> >  
> >  		rx_packets += stats->rx_packets;
> > @@ -98,7 +99,8 @@ static void appldata_get_net_sum_data(vo
> >  		collisions += stats->collisions;
> >  		i++;
> >  	}
> > -	read_unlock(&dev_base_lock);
> > +	rcu_read_unlock();
> > +
> >  	net_data->nr_interfaces = i;
> >  	net_data->rx_packets = rx_packets;
> >  	net_data->tx_packets = tx_packets;
> > 
> 
> Not sure if dev_get_stats(dev) could sleep on some devices...
> 

It would have already been broken since dev_get_stats is previously
called with read_lock(), and sleeping with any lock held causes warning.

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