[3.13.y.z extended stable] Patch "udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.6.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From d34126277484f446b7933a59fe7156e4c6e690c8 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <edumazet at google.com>
Date: Thu, 12 Jun 2014 16:13:06 -0700
Subject: udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

[ Upstream commit 63c6f81cdde58c41da62a8d8a209592e42a0203e ]

Its too easy to add thousand of UDP sockets on a particular bucket,
and slow down an innocent multicast receiver.

Early demux is supposed to be an optimization, we should avoid spending
too much time in it.

It is interesting to note __udp4_lib_demux_lookup() only tries to
match first socket in the chain.

10 is the threshold we already have in __udp4_lib_lookup() to switch
to secondary hash.

Fixes: 421b3885bf6d5 ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reported-by: David Held <drheld at google.com>
Cc: Shawn Bohrer <sbohrer at rgmadvisors.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/ipv4/udp.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index a7e4729..98ea36d 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1833,6 +1833,10 @@ static struct sock *__udp4_lib_mcast_demux_lookup(struct net *net,
 	unsigned int count, slot = udp_hashfn(net, hnum, udp_table.mask);
 	struct udp_hslot *hslot = &udp_table.hash[slot];

+	/* Do not bother scanning a too big list */
+	if (hslot->count > 10)
+		return NULL;
+
 	rcu_read_lock();
 begin:
 	count = 0;
--
1.9.1