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

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Message-ID: <20081029163739.GB6732@linux.vnet.ibm.com>
Date:	Wed, 29 Oct 2008 09:37:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Corey Minyard <minyard@....org>,
	David Miller <davem@...emloft.net>, shemminger@...tta.com,
	benny+usenet@...rsen.dk, netdev@...r.kernel.org,
	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.

On Wed, Oct 29, 2008 at 05:09:53PM +0100, Eric Dumazet wrote:
> Corey Minyard a écrit :
>> 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.
>
> Not sure about this one Corey.
>
> If a reader catches previous value of item->sk_hash, two cases are to be 
> taken into :
>
> 1) its udp_hashfn(net, sk->sk_hash) is != hash   -> goto begin : Reader 
> will redo its scan
>
> 2) its udp_hashfn(net, sk->sk_hash) is == hash
>  -> next pointer is good enough : it points to next item in same hash 
> chain.
>     No need to rescan the chain at this point.
>     Yes we could miss the fact that a new port was bound and this UDP 
> message could be lost.

3) its udp_hashfn(net, sk-sk_hash) is == hash, but only because it was
removed, freed, reallocated, and then readded with the same hash value,
possibly carrying the reader to a new position in the same list.

You might well cover this (will examine your code in detail on my plane
flight starting about 20 hours from now), but thought I should point it
out.  ;-)

						Thanx, Paul

> If we force a smp_wmb(), reader would fetch pointer to begin of list.
>
>
> 1) its udp_hashfn(net, sk->sk_hash) is != hash   -> goto begin : Reader 
> will redo its scan : next pointer value had no meaning
>
> 2) its udp_hashfn(net, sk->sk_hash) is == hash
>  ->next pointer "force reader" to rescan the chain, but wont find new 
> items.
>
> In any case, we cannot lost an UDP message sent to a stable port 
> (previously bound)
>
>
> Thanks
>
> --
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