Patch "bridge: disable softirqs around br_fdb_update to avoid lockup" has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

bridge: disable softirqs around br_fdb_update to avoid lockup

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bridge-disable-softirqs-around-br_fdb_update-to-avoid-lockup.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Nikolay Aleksandrov <[email protected]>
Date: Sat, 6 Jun 2015 06:49:00 -0700
Subject: bridge: disable softirqs around br_fdb_update to avoid lockup

From: Nikolay Aleksandrov <[email protected]>

[ Upstream commit c4c832f89dc468cf11dc0dd17206bace44526651 ]

br_fdb_update() can be called in process context in the following way:
br_fdb_add() -> __br_fdb_add() -> br_fdb_update() (if NTF_USE flag is set) so we need to disable softirqs because there are softirq users of the hash_lock. One easy way to reproduce this is to modify the bridge utility to set NTF_USE, enable stp and then set maxageing to a low value so br_fdb_cleanup() is called frequently and then just add new entries in a loop. This happens because br_fdb_cleanup() is called from timer/softirq context. The spin locks in br_fdb_update were _bh before commit f8ae737deea1 ("[BRIDGE]: forwarding remove unneeded preempt and bh diasables") and at the time that commit was correct because br_fdb_update() couldn't be called from process context, but that changed after commit:
292d1398983f ("bridge: add NTF_USE support")
Using local_bh_disable/enable around br_fdb_update() allows us to keep using the spin_lock/unlock in br_fdb_update for the fast-path.

---
 net/bridge/br_fdb.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -796,9 +796,11 @@ static int __br_fdb_add(struct ndmsg *nd
 	int err = 0;
 
 	if (ndm->ndm_flags & NTF_USE) {
+		local_bh_disable();
 		rcu_read_lock();
 		br_fdb_update(p->br, p, addr, vid, true);
 		rcu_read_unlock();
+		local_bh_enable();
 	} else {
 		spin_lock_bh(&p->br->hash_lock);
 		err = fdb_add_entry(p, addr, ndm->ndm_state,

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/bridge-disable-softirqs-around-br_fdb_update-to-avoid-lockup.patch