Patch "net: fix races in netdev_tx_sent_queue()/dev_watchdog()" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

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

    net: fix races in netdev_tx_sent_queue()/dev_watchdog()

to the 6.6-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:
     net-fix-races-in-netdev_tx_sent_queue-dev_watchdog.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit ef2042b8df5bc2848ca6b65cc4f804b13e0cdc38
Author: Eric Dumazet <edumazet@xxxxxxxxxx>
Date:   Tue Oct 15 19:41:18 2024 +0000

    net: fix races in netdev_tx_sent_queue()/dev_watchdog()
    
    [ Upstream commit 95ecba62e2fd201bcdcca636f5d774f1cd4f1458 ]
    
    Some workloads hit the infamous dev_watchdog() message:
    
    "NETDEV WATCHDOG: eth0 (xxxx): transmit queue XX timed out"
    
    It seems possible to hit this even for perfectly normal
    BQL enabled drivers:
    
    1) Assume a TX queue was idle for more than dev->watchdog_timeo
       (5 seconds unless changed by the driver)
    
    2) Assume a big packet is sent, exceeding current BQL limit.
    
    3) Driver ndo_start_xmit() puts the packet in TX ring,
       and netdev_tx_sent_queue() is called.
    
    4) QUEUE_STATE_STACK_XOFF could be set from netdev_tx_sent_queue()
       before txq->trans_start has been written.
    
    5) txq->trans_start is written later, from netdev_start_xmit()
    
        if (rc == NETDEV_TX_OK)
              txq_trans_update(txq)
    
    dev_watchdog() running on another cpu could read the old
    txq->trans_start, and then see QUEUE_STATE_STACK_XOFF, because 5)
    did not happen yet.
    
    To solve the issue, write txq->trans_start right before one XOFF bit
    is set :
    
    - _QUEUE_STATE_DRV_XOFF from netif_tx_stop_queue()
    - __QUEUE_STATE_STACK_XOFF from netdev_tx_sent_queue()
    
    From dev_watchdog(), we have to read txq->state before txq->trans_start.
    
    Add memory barriers to enforce correct ordering.
    
    In the future, we could avoid writing over txq->trans_start for normal
    operations, and rename this field to txq->xoff_start_time.
    
    Fixes: bec251bc8b6a ("net: no longer stop all TX queues in dev_watchdog()")
    Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Reviewed-by: Willem de Bruijn <willemb@xxxxxxxxxx>
    Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
    Link: https://patch.msgid.link/20241015194118.3951657-1-edumazet@xxxxxxxxxx
    Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 8f5ac20b4c03d..1576e7443eee5 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3363,6 +3363,12 @@ static inline void netif_tx_wake_all_queues(struct net_device *dev)
 
 static __always_inline void netif_tx_stop_queue(struct netdev_queue *dev_queue)
 {
+	/* Paired with READ_ONCE() from dev_watchdog() */
+	WRITE_ONCE(dev_queue->trans_start, jiffies);
+
+	/* This barrier is paired with smp_mb() from dev_watchdog() */
+	smp_mb__before_atomic();
+
 	/* Must be an atomic op see netif_txq_try_stop() */
 	set_bit(__QUEUE_STATE_DRV_XOFF, &dev_queue->state);
 }
@@ -3479,6 +3485,12 @@ static inline void netdev_tx_sent_queue(struct netdev_queue *dev_queue,
 	if (likely(dql_avail(&dev_queue->dql) >= 0))
 		return;
 
+	/* Paired with READ_ONCE() from dev_watchdog() */
+	WRITE_ONCE(dev_queue->trans_start, jiffies);
+
+	/* This barrier is paired with smp_mb() from dev_watchdog() */
+	smp_mb__before_atomic();
+
 	set_bit(__QUEUE_STATE_STACK_XOFF, &dev_queue->state);
 
 	/*
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 7f0c8df7b63e0..b51af871a621c 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -511,9 +511,15 @@ static void dev_watchdog(struct timer_list *t)
 				struct netdev_queue *txq;
 
 				txq = netdev_get_tx_queue(dev, i);
-				trans_start = READ_ONCE(txq->trans_start);
 				if (!netif_xmit_stopped(txq))
 					continue;
+
+				/* Paired with WRITE_ONCE() + smp_mb...() in
+				 * netdev_tx_sent_queue() and netif_tx_stop_queue().
+				 */
+				smp_mb();
+				trans_start = READ_ONCE(txq->trans_start);
+
 				if (time_after(jiffies, trans_start + dev->watchdog_timeo)) {
 					timedout_ms = jiffies_to_msecs(jiffies - trans_start);
 					atomic_long_inc(&txq->trans_timeout);