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


From: Nithin Sujir <nsu...@tintri.com>

[ 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

Signed-off-by: Nithin Nayak Sujir <nsu...@tintri.com>
Cc: Mahesh Bandewar <mahe...@google.com>
Cc: Jay Vosburgh <jay.vosbu...@canonical.com>
Acked-by: Mahesh Bandewar <mahe...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Sasha Levin <alexander.le...@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
 drivers/net/bonding/bond_main.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2605,11 +2605,13 @@ 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
@@ -2636,7 +2638,7 @@ 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)
@@ -2667,6 +2669,11 @@ 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) {
                        if (BOND_MODE(bond) == BOND_MODE_XOR)

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