[PATCH 3.11 195/208] bridge: use spin_lock_bh() in br_multicast_set_hash_max

[Luis Henriques]

3.11.10.3 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Curt Brune <curt at cumulusnetworks.com>

commit fe0d692bbc645786bce1a98439e548ae619269f5 upstream.

br_multicast_set_hash_max() is called from process context in
net/bridge/br_sysfs_br.c by the sysfs store_hash_max() function.

br_multicast_set_hash_max() calls spin_lock(&br->multicast_lock),
which can deadlock the CPU if a softirq that also tries to take the
same lock interrupts br_multicast_set_hash_max() while the lock is
held .  This can happen quite easily when any of the bridge multicast
timers expire, which try to take the same lock.

The fix here is to use spin_lock_bh(), preventing other softirqs from
executing on this CPU.

Steps to reproduce:

1. Create a bridge with several interfaces (I used 4).
2. Set the "multicast query interval" to a low number, like 2.
3. Enable the bridge as a multicast querier.
4. Repeatedly set the bridge hash_max parameter via sysfs.

  # brctl addbr br0
  # brctl addif br0 eth1 eth2 eth3 eth4
  # brctl setmcqi br0 2
  # brctl setmcquerier br0 1

  # while true ; do echo 4096 > /sys/class/net/br0/bridge/hash_max; done

Signed-off-by: Curt Brune <curt at cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma at cumulusnetworks.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 net/bridge/br_multicast.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index fbad619..b02fb47 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1997,7 +1997,7 @@ int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)
 	u32 old;
 	struct net_bridge_mdb_htable *mdb;
 
-	spin_lock(&br->multicast_lock);
+	spin_lock_bh(&br->multicast_lock);
 	if (!netif_running(br->dev))
 		goto unlock;
 
@@ -2029,7 +2029,7 @@ rollback:
 	}
 
 unlock:
-	spin_unlock(&br->multicast_lock);
+	spin_unlock_bh(&br->multicast_lock);
 
 	return err;
 }
-- 
1.8.3.2