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

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Message-ID: <20091110182436.GA13374@fogou.chygwyn.com>
Date:	Tue, 10 Nov 2009 18:24:36 +0000
From:	steve@...gwyn.com
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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

Hi,

On Tue, Nov 10, 2009 at 10:50:53AM -0800, Stephen Hemminger wrote:
> 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.

The original author has tried to remember what he was thinking when
he wrote this code :-)

I think you are right that it should be rtnl_lock() as we don't want
the address list changing at this point. On the other hand I notice
also that other bits of the code seem to be using dev_base_lock too.

Maybe this is a hang over from another era? I'll have to refresh
my memory some more before I can give a verdict on that I'm afraid,

Steve.

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