netdev - Re: why does there need to lock?

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Message-ID: <1269245126.3029.3.camel@edumazet-laptop>
Date:	Mon, 22 Mar 2010 09:05:26 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	杨硕 <shanqn@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: why does there need to lock?

Le lundi 22 mars 2010 à 08:41 +0800, 杨硕 a écrit :
> Hi, i'm confused about why does "rcu_assign_pointer(dev->ip_ptr,
> in_dev);" need to rcu lock?
> 

I am confused by your question

rcu_assign_pointer doesnt need rcu lock.

It only makes sure all previous memory changes are committed to memory
before the "dev->ip_ptr = in_dev;" assignement, so that concurrent
readers cannot find the new pointer and read previous values of
dev->ip_ptr->fields


> TIA :)
> 
> static struct in_device *inetdev_init(struct net_device *dev)
> {
> 	struct in_device *in_dev;
> 
> 	ASSERT_RTNL();
> 
> 	in_dev = kzalloc(sizeof(*in_dev), GFP_KERNEL);
> 	if (!in_dev)
> 		goto out;
> 	memcpy(&in_dev->cnf, dev_net(dev)->ipv4.devconf_dflt,
> 			sizeof(in_dev->cnf));
> 	in_dev->cnf.sysctl = NULL;
> 	in_dev->dev = dev;
> 	if ((in_dev->arp_parms = neigh_parms_alloc(dev, &arp_tbl)) == NULL)
> 		goto out_kfree;
> 	if (IPV4_DEVCONF(in_dev->cnf, FORWARDING))
> 		dev_disable_lro(dev);
> 	/* Reference in_dev->dev */
> 	dev_hold(dev);
> 	/* Account for reference dev->ip_ptr (below) */
> 	in_dev_hold(in_dev);
> 
> 	devinet_sysctl_register(in_dev);
> 	ip_mc_init_dev(in_dev);
> 	if (dev->flags & IFF_UP)
> 		ip_mc_up(in_dev);
> 
> 	/* we can receive as soon as ip_ptr is set -- do this last */
> 	rcu_assign_pointer(dev->ip_ptr, in_dev);
> out:
> 	return in_dev;
> out_kfree:
> 	kfree(in_dev);
> 	in_dev = NULL;
> 	goto out;
> }


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