Commit 95ecba62 authored by Eric Dumazet's avatar Eric Dumazet Committed by Paolo Abeni
Browse files

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



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: bec251bc ("net: no longer stop all TX queues in dev_watchdog()")
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarWillem de Bruijn <willemb@google.com>
Reviewed-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20241015194118.3951657-1-edumazet@google.com


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent 47dd5447

include/linux/netdevice.h

+12 −0
Original line number Diff line number Diff line
@@ -3325,6 +3325,12 @@ 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);

}
@@ -3451,6 +3457,12 @@ 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);



	/*

net/sched/sch_generic.c

+7 −1
Original line number Diff line number Diff line
@@ -512,9 +512,15 @@ 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);