[PATCH net v3] bonding: fix missing down path on speed/duplex failure after FAIL->UP

[Timur Stukan <timur.stukan.linux@gmail.com>]

In bond_miimon_commit(), when a slave is proposed as BOND_LINK_UP but
bond_update_speed_duplex() fails in bonding mode that needs speed/duplex
(802.3ad, tlb, alb), the slave is set to BOND_LINK_DOWN directly. This
bypasses bond_set_slave_link_state(), bond_miimon_link_change() and
failover.

This is fine for slave that coming from BOND_LINK_BACK, which was never
brought up and is not in use. But since commit 1899bb325149 ("bonding:
fix state transition issue in link monitoring") the commit phase also
handles BOND_LINK_FAIL -> BOND_LINK_UP which means a slave that is still
in use and may be curr_active_slave.

If the link recovers within downdelay and goes down again before the
commit phase reads speed/duplex, the slave is silently marked down while
remaining the active slave in worst cases. In alb/tlb the bond MAC stays
on the dead port, no failover happens and TLB/RLB state is not cleaned,
so the bond stops passing traffic until that link comes back even though
other slaves are up. A non-active slave is affected too, in alb, RLB
clients assigned to it are not reassigned and become unreachable.

Reproduced in tlb and alb mode with miimon=100 and downdelay=10000 by
flapping a slave within downdelay and dropping it again before commit
(a longer delay was added between inspect and commit to make window
wider and easier to test):
  bond0: (slave enp0s3): link status down for interface, disabling it in 10000 ms
  bond0: (slave enp0s3): link status up again after 3400 ms
  bond0: (slave enp0s3): failed to get link speed/duplex

procfs then shows the active slave with "MII Status: down" and "Link
Failure Count: 0", and all traffic is lost.

Fix this by dropping the loop in bond_mii_monitor() that applied all
proposed link states before bond_miimon_commit(). bond_miimon_commit()
now applies each state itself: UP after the speed/duplex check, as it
already did, DOWN in the down path, and FAIL/BACK directly. So when the
speed/duplex check runs, slave->link still holds the previous state. If
the check fails for a slave that is FAIL or UP, take it through the
regular down path, including failover. DOWN and BACK slaves were
never brought up and are still just reverted to DOWN. The down path is
moved into bond_miimon_slave_down() so both callers share it.

After the fix:
  bond0: (slave enp0s3): failed to get link speed/duplex
  bond0: (slave enp0s3): link status definitely down, disabling slave
  bond0: (slave enp0s4): making interface the new active one

Fixes: 1899bb325149 ("bonding: fix state transition issue in link monitoring")
Signed-off-by: Timur Stukan <timur.stukan.linux@gmail.com>
---

Notes:
    Changes in v3:
    - Drop the loop in bond_mii_monitor() and apply FAIL/BACK in
      bond_miimon_commit() with bond_commit_link_state() (Jay, Hangbin)
    - Also take the full down path for a slave that is already UP when
      the speed/duplex update fails (Sashiko)
    - The 802.3ad notifier issue reported by Sashiko is fixed separately:
      https://lore.kernel.org/netdev/20261101145653.63648-1-timur.stukan.linux@gmail.com/
    - v2: https://lore.kernel.org/netdev/20261028104758.299129-1-timur.stukan.linux@gmail.com/
    
    Changes in v2:
    - Drop link_prev_state. Instead, don't commit UP proposals in
      bond_mii_monitor() and let bond_miimon_commit() commit them after
      the speed/duplex check, so no extra state is needed (Jay)
    - v1: https://lore.kernel.org/netdev/20261022103821.89552-1-timur.stukan.linux@gmail.com/

 drivers/net/bonding/bond_main.c | 63 +++++++++++++++++++--------------
 1 file changed, 37 insertions(+), 26 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 7019e5d8fdc8..d6b6c125470f 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -2779,6 +2779,27 @@ static void bond_miimon_link_change(struct bonding *bond,
 	}
 }
 
+static bool bond_miimon_slave_down(struct bonding *bond,
+				   struct slave *slave)
+{
+	if (slave->link_failure_count < UINT_MAX)
+		slave->link_failure_count++;
+
+	bond_set_slave_link_state(slave, BOND_LINK_DOWN,
+				  BOND_SLAVE_NOTIFY_NOW);
+
+	if (BOND_MODE(bond) == BOND_MODE_ACTIVEBACKUP ||
+	    BOND_MODE(bond) == BOND_MODE_8023AD)
+		bond_set_slave_inactive_flags(slave,
+					      BOND_SLAVE_NOTIFY_NOW);
+
+	slave_info(bond->dev, slave->dev, "link status definitely down, disabling slave\n");
+
+	bond_miimon_link_change(bond, slave, BOND_LINK_DOWN);
+
+	return slave == rcu_access_pointer(bond->curr_active_slave);
+}
+
 static void bond_miimon_commit(struct bonding *bond)
 {
 	struct slave *slave, *primary, *active;
@@ -2807,10 +2828,22 @@ static void bond_miimon_commit(struct bonding *bond)
 			err = bond_update_speed_duplex(slave);
 			netdev_unlock_ops(slave->dev);
 			if (err && bond_needs_speed_duplex(bond)) {
-				slave->link = BOND_LINK_DOWN;
 				if (net_ratelimit())
 					slave_warn(bond->dev, slave->dev,
 						   "failed to get link speed/duplex\n");
+				/* Coming from FAIL/UP, the slave may still be
+				 * in use and may be curr_active_slave, so take
+				 * it through the full down path. Coming from
+				 * DOWN/BACK, it was never brought up, so just
+				 * revert it to DOWN.
+				 */
+				if (slave->link == BOND_LINK_FAIL ||
+				    slave->link == BOND_LINK_UP) {
+					if (bond_miimon_slave_down(bond, slave))
+						do_failover = true;
+					continue;
+				}
+				slave->link = BOND_LINK_DOWN;
 				continue;
 			}
 			bond_set_slave_link_state(slave, BOND_LINK_UP,
@@ -2839,28 +2872,13 @@ static void bond_miimon_commit(struct bonding *bond)
 			continue;
 
 		case BOND_LINK_DOWN:
-			if (slave->link_failure_count < UINT_MAX)
-				slave->link_failure_count++;
-
-			bond_set_slave_link_state(slave, BOND_LINK_DOWN,
-						  BOND_SLAVE_NOTIFY_NOW);
-
-			if (BOND_MODE(bond) == BOND_MODE_ACTIVEBACKUP ||
-			    BOND_MODE(bond) == BOND_MODE_8023AD)
-				bond_set_slave_inactive_flags(slave,
-							      BOND_SLAVE_NOTIFY_NOW);
-
-			slave_info(bond->dev, slave->dev, "link status definitely down, disabling slave\n");
-
-			bond_miimon_link_change(bond, slave, BOND_LINK_DOWN);
-
-			if (slave == rcu_access_pointer(bond->curr_active_slave))
+			if (bond_miimon_slave_down(bond, slave))
 				do_failover = true;
-
 			continue;
 
 		case BOND_LINK_FAIL:
 		case BOND_LINK_BACK:
+			bond_commit_link_state(slave, BOND_SLAVE_NOTIFY_LATER);
 			slave_dbg(bond->dev, slave->dev, "link_new_state %d on slave\n",
 				  slave->link_new_state);
 			continue;
@@ -2894,8 +2912,6 @@ static void bond_mii_monitor(struct work_struct *work)
 {
 	struct bonding *bond = container_of(work, struct bonding,
 					    mii_work.work);
-	struct list_head *iter;
-	struct slave *slave;
 	unsigned long delay;
 	bool commit;
 
@@ -2917,13 +2933,8 @@ static void bond_mii_monitor(struct work_struct *work)
 			goto re_arm;
 		}
 
-		if (commit) {
-			bond_for_each_slave(bond, slave, iter) {
-				bond_commit_link_state(slave,
-						       BOND_SLAVE_NOTIFY_LATER);
-			}
+		if (commit)
 			bond_miimon_commit(bond);
-		}
 
 		if (bond->send_peer_notif)
 			bond_peer_notify_may_events(bond, true);
-- 
2.43.0