6.6-stable review patch. If anyone has any objections, please let me know.
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From: Eric Dumazet edumazet@google.com
[ 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@google.com Reviewed-by: Willem de Bruijn willemb@google.com Reviewed-by: Toke Høiland-Jørgensen toke@redhat.com Link: https://patch.msgid.link/20241015194118.3951657-1-edumazet@google.com Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/netdevice.h | 12 ++++++++++++ net/sched/sch_generic.c | 8 +++++++- 2 files changed, 19 insertions(+), 1 deletion(-)
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);