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Message-ID: <20081029181114.GC6732@linux.vnet.ibm.com>
Date: Wed, 29 Oct 2008 11:11:14 -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 06:32:29PM +0100, Eric Dumazet wrote:
> Paul E. McKenney a écrit :
>> 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.
>
> yes, but 'new position' is 'before any not yet examined objects', since
> we insert objects only at chain head.
OK. However, this reasoning assumes that a socket with a given
udp_hashfn() value will appear on one and only one list. There are no
side lists for sockets in other states? (listen, &c)
>> 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. ;-)
>
> Yes, I'll double check too, this seems tricky :)
;-)
> About SLAB_DESTROY_BY_RCU effect, we now have two different kmem_cache for
> "UDP-Lite"
> and "UDP".
>
> This is expected, but we could avoid that and alias these caches, since
> these objects have the same *type* . (The fields used for the RCU lookups,
> deletes and inserts are the same)
>
> Maybe a hack in net/ipv4/udplite.c before calling proto_register(), to
> copy the kmem_cache from UDP.
As long as this preserves the aforementioned assumption that a socket
with a given hash can appear on one and only one list. ;-)
Thanx, Paul
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