bridge: use spin_lock_bh() in br_multicast_set_hash_max

Curt Brune · davem330 · commit fe0d692bbc64 · 2014-01-06T16:39:47.000-05:00
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(&amp;br-&gt;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 &gt; /sys/class/net/br0/bridge/hash_max; done

Signed-off-by: Curt Brune &lt;curt@cumulusnetworks.com&gt;
Signed-off-by: Scott Feldman &lt;sfeldma@cumulusnetworks.com&gt;
Signed-off-by: David S. Miller &lt;davem@davemloft.net&gt;
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
@@ -1998,7 +1998,7 @@ 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;
 
@@ -2030,7 +2030,7 @@ int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)
 	}
 
 unlock:
-	spin_unlock(&br->multicast_lock);
+	spin_unlock_bh(&br->multicast_lock);
 
 	return err;
 }

Original file line number	Diff line number	Diff line change
`@@ -1998,7 +1998,7 @@ int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)`
`1998`	`1998`	`u32 old;`
`1999`	`1999`	`struct net_bridge_mdb_htable *mdb;`
`2000`	`2000`
`2001`		`- spin_lock(&br->multicast_lock);`
	`2001`	`+ spin_lock_bh(&br->multicast_lock);`
`2002`	`2002`	`if (!netif_running(br->dev))`
`2003`	`2003`	`goto unlock;`
`2004`	`2004`
`@@ -2030,7 +2030,7 @@ int br_multicast_set_hash_max(struct net_bridge *br, unsigned long val)`
`2030`	`2030`	`}`
`2031`	`2031`
`2032`	`2032`	`unlock:`
`2033`		`- spin_unlock(&br->multicast_lock);`
	`2033`	`+ spin_unlock_bh(&br->multicast_lock);`
`2034`	`2034`
`2035`	`2035`	`return err;`
`2036`	`2036`	`}`