netdev - Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

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Date:	Wed, 29 Oct 2008 10:34:32 -0500
From:	Corey Minyard <minyard@....org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	benny+usenet@...rsen.dk, netdev@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com,
	Christoph Lameter <cl@...ux-foundation.org>,
	a.p.zijlstra@...llo.nl, johnpol@....mipt.ru,
	Christian Bell <christian@...i.com>
Subject: Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

Eric Dumazet wrote:
> Corey Minyard found a race added in commit 
> 271b72c7fa82c2c7a795bc16896149933110672d
> (udp: RCU handling for Unicast packets.)
>
> "If the socket is moved from one list to another list in-between the 
> time  the hash is calculated and the next field is accessed, and the 
> socket  has moved to the end of the new list, the traversal will not 
> complete  properly on the list it should have, since the socket will 
> be on the end  of the new list and there's not a way to tell it's on a 
> new list and  restart the list traversal.  I think that this can be 
> solved by  pre-fetching the "next" field (with proper barriers) before 
> checking the  hash."
>
> This patch corrects this problem, introducing a new 
> sk_for_each_rcu_safenext()
> macro.
You also need the appropriate smp_wmb() in udp_lib_get_port() after 
sk_hash is set, I think, so the next field is guaranteed to be changed 
after the hash value is changed.


-corey
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