[PATCH] udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

[PATCH] udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

[Eric Dumazet]

From: Eric Dumazet <edumazet@google.com>

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@google.com>
Reported-by: David Held <drheld@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
---
 net/ipv4/udp.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 185ed3e59802..7064dbfa8101 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1861,6 +1861,10 @@ 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;

Re: [PATCH] udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

[David Miller]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 12 Jun 2014 16:13:06 -0700

> From: Eric Dumazet <edumazet@google.com>
> 
> 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@google.com>
> Reported-by: David Held <drheld@google.com>

Applied and queued up for -stable, thanks Eric.