netdev - Re: [PATCH 07/10] decnet: use RCU to find network devices

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Message-ID: <20091110105053.16687e93@nehalam>
Date:	Tue, 10 Nov 2009 10:50:53 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Christine Caulfield <christine.caulfield@...glemail.com>,
	Hannes Eder <hannes@...neseder.net>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	netdev@...r.kernel.org, linux-decnet-users@...ts.sourceforge.net
Subject: Re: [PATCH 07/10] decnet: use RCU to find network devices

On Tue, 10 Nov 2009 19:43:21 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Stephen Hemminger a écrit :
> > When showing device statistics use RCU rather than read_lock(&dev_base_lock)
> > Compile tested only.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> > 
> > --- a/net/decnet/dn_dev.c	2009-11-10 09:30:55.557376454 -0800
> > +++ b/net/decnet/dn_dev.c	2009-11-10 09:40:03.847005394 -0800
> > @@ -856,9 +856,7 @@ int dn_dev_bind_default(__le16 *addr)
> >  	dev = dn_dev_get_default();
> >  last_chance:
> >  	if (dev) {
> > -		read_lock(&dev_base_lock);
> >  		rv = dn_dev_get_first(dev, addr);
> > -		read_unlock(&dev_base_lock);
> >  		dev_put(dev);
> >  		if (rv == 0 || dev == init_net.loopback_dev)
> >  			return rv;
> > @@ -1323,18 +1321,18 @@ static inline int is_dn_dev(struct net_d
> 
> 
> I dont understand this part. Why previous locking can be avoided ?

dn_dev_get_default acquires a reference on dev so the device can
not go away.

It could be the original author meant to ensure the address list
doesn't change. If so, then rtnl_lock() should have been used.
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