bridge: Add core IGMP snooping support

This patch adds the core functionality of IGMP snooping support
without actually hooking it up.  So this patch should be a no-op
as far as the bridge's external behaviour is concerned.

All the new code and data is controlled by the Kconfig option
BRIDGE_IGMP_SNOOPING.  A run-time toggle is also available.

The multicast switching is done using an hash table that is
lockless on the read-side through RCU.  On the write-side the
new multicast_lock is used for all operations.  The hash table
supports dynamic growth/rehashing.

The hash table will be rehashed if any chain length exceeds a
preset limit.  If rehashing does not reduce the maximum chain
length then snooping will be disabled.

These features may be added in future (in no particular order):

* IGMPv3 source support
* Non-querier router detection
* IPv6

Signed-off-by: Herbert Xu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

net/bridge/Kconfig

@@ -31,3 +31,15 @@ config BRIDGE
 	  will be called bridge.
 
 	  If unsure, say N.
+
+config BRIDGE_IGMP_SNOOPING
+	bool "IGMP snooping"
+	default y
+	---help---
+	  If you say Y here, then the Ethernet bridge will be able selectively
+	  forward multicast traffic based on IGMP traffic received from each
+	  port.
+
+	  Say N to exclude this support and reduce the binary size.
+
+	  If unsure, say Y.

net/bridge/Makefile

@@ -12,4 +12,6 @@ bridge-$(CONFIG_SYSFS) += br_sysfs_if.o br_sysfs_br.o
 
 bridge-$(CONFIG_BRIDGE_NETFILTER) += br_netfilter.o
 
+bridge-$(CONFIG_BRIDGE_IGMP_SNOOPING) += br_multicast.o
+
 obj-$(CONFIG_BRIDGE_NF_EBTABLES) += netfilter/