[PATCH 3.8 166/166] bridge: use spin_lock_bh() in br_multicast_set_hash_max

Kamal Mostafa
2014-01-15T21:53:00Z

From: Curt Brune <curt@cumulusnetworks.com>

[ Upstream commit fe0d692bbc645786bce1a98439e548ae619269f5 ]

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@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Kamal Mostafa <kamal@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 4a51874..fda703b 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1812,7 +1812,7 @@ 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;

@@ -1844,7 +1844,7 @@ rollback:
}

unlock:
- spin_unlock(&br->multicast_lock);
+ spin_unlock_bh(&br->multicast_lock);

return err;
}
--
1.8.3.2