Patch "bonding: Don't update slave->link until ready to commit" has been added to the 4.4-stable tree

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

    bonding: Don't update slave->link until ready to commit

to the 4.4-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:
bonding-don-t-update-slave-link-until-ready-to-commit.patch
and it can be found in the queue-4.4 subdirectory.

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

From: Nithin Sujir <[email protected]>
Date: Wed, 24 May 2017 19:45:17 -0700
Subject: bonding: Don't update slave->link until ready to commit

From: Nithin Sujir <[email protected]>

[ Upstream commit 797a93647a48d6cb8a20641a86a71713a947f786 ]

In the loadbalance arp monitoring scheme, when a slave link change is detected, the slave->link is immediately updated and slave_state_changed is set. Later down the function, the rtnl_lock is acquired and the changes are committed, updating the bond link state.

However, the acquisition of the rtnl_lock can fail. The next time the monitor runs, since slave->link is already updated, it determines that link is unchanged. This results in the bond link state permanently out of sync with the slave link.

This patch modifies bond_loadbalance_arp_mon() to handle link changes identical to bond_ab_arp_{inspect/commit}(). The new link state is maintained in slave->new_link until we're ready to commit at which point it's copied into slave->link.

NOTE: miimon_{inspect/commit}() has a more complex state machine requiring the use of the bond_{propose,commit}_link_state() functions which maintains the intermediate state in slave->link_new_state. The arp monitors don't require that.

Testing: This bug is very easy to reproduce with the following steps.
1. In a loop, toggle a slave link of a bond slave interface.
2. In a separate loop, do ifconfig up/down of an unrelated interface to create contention for rtnl_lock.
Within a few iterations, the bond link goes out of sync with the slave link.

---
 drivers/net/bonding/bond_main.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

@@ -2555,11 +2555,13 @@ static void bond_loadbalance_arp_mon(str
 	bond_for_each_slave_rcu(bond, slave, iter) {
 		unsigned long trans_start = dev_trans_start(slave->dev);
 
+		slave->new_link = BOND_LINK_NOCHANGE;
+
 		if (slave->link != BOND_LINK_UP) {
 			if (bond_time_in_interval(bond, trans_start, 1) &&
 			    bond_time_in_interval(bond, slave->last_rx, 1)) {
 
-				slave->link  = BOND_LINK_UP;
+				slave->new_link = BOND_LINK_UP;
 				slave_state_changed = 1;
 
 				/* primary_slave has no meaning in round-robin

@@ -2586,7 +2588,7 @@ static void bond_loadbalance_arp_mon(str
 			if (!bond_time_in_interval(bond, trans_start, 2) ||
 			    !bond_time_in_interval(bond, slave->last_rx, 2)) {
 
-				slave->link  = BOND_LINK_DOWN;
+				slave->new_link = BOND_LINK_DOWN;
 				slave_state_changed = 1;
 
 				if (slave->link_failure_count < UINT_MAX)

@@ -2617,6 +2619,11 @@ static void bond_loadbalance_arp_mon(str
 		if (!rtnl_trylock())
 			goto re_arm;
 
+		bond_for_each_slave(bond, slave, iter) {
+			if (slave->new_link != BOND_LINK_NOCHANGE)
+				slave->link = slave->new_link;
+		}
+
 		if (slave_state_changed) {
 			bond_slave_state_change(bond);
 			if (BOND_MODE(bond) == BOND_MODE_XOR)

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

queue-4.4/bonding-don-t-update-slave-link-until-ready-to-commit.patch