netdev - Re: [Patch net] ipv6,mcast: always hold idev->lock before mca

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Message-Id: <20130511.161158.1119064774406426541.davem@davemloft.net>
Date:	Sat, 11 May 2013 16:11:58 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	amwang@...hat.com
Cc:	netdev@...r.kernel.org, dingtianhong@...wei.com,
	yoshfuji@...ux-ipv6.org
Subject: Re: [Patch net] ipv6,mcast: always hold idev->lock before mca_lock

From: Cong Wang <amwang@...hat.com>
Date: Wed,  8 May 2013 15:41:54 +0800

> @@ -1369,7 +1370,7 @@ static struct sk_buff *mld_newpack(struct net_device *dev, int size)
>  
>  	skb_reserve(skb, hlen);
>  
> -	if (ipv6_get_lladdr(dev, &addr_buf, IFA_F_TENTATIVE)) {
> +	if (__ipv6_get_lladdr(idev, &addr_buf, IFA_F_TENTATIVE)) {
>  		/* <draft-ietf-magma-mld-source-05.txt>:
>  		 * use unspecified address as the source address
>  		 * when a valid link-local address is not available.

You aren't necessarily going to be holding idev->lock, therefore you can't
just do a lockless traversal of idev->addr_list here.

Yes, you can elide the rcu_read_lock() because you have a known reference
to 'idev' in these paths, but you can't get rid of the address list locking
altogether.
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