6.12-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jesper Dangaard Brouer hawk@kernel.org
[ Upstream commit 5442a9da69789741bfda39f34ee7f69552bf0c56 ]
Commit dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops") introduced a race condition that can lead to a permanently stalled TXQ. This was observed in production on ARM64 systems (Ampere Altra Max).
The race occurs in veth_xmit(). The producer observes a full ptr_ring and stops the queue (netif_tx_stop_queue()). The subsequent conditional logic, intended to re-wake the queue if the consumer had just emptied it (if (__ptr_ring_empty(...)) netif_tx_wake_queue()), can fail. This leads to a "lost wakeup" where the TXQ remains stopped (QUEUE_STATE_DRV_XOFF) and traffic halts.
This failure is caused by an incorrect use of the __ptr_ring_empty() API from the producer side. As noted in kernel comments, this check is not guaranteed to be correct if a consumer is operating on another CPU. The empty test is based on ptr_ring->consumer_head, making it reliable only for the consumer. Using this check from the producer side is fundamentally racy.
This patch fixes the race by adopting the more robust logic from an earlier version V4 of the patchset, which always flushed the peer:
(1) In veth_xmit(), the racy conditional wake-up logic and its memory barrier are removed. Instead, after stopping the queue, we unconditionally call __veth_xdp_flush(rq). This guarantees that the NAPI consumer is scheduled, making it solely responsible for re-waking the TXQ. This handles the race where veth_poll() consumes all packets and completes NAPI *before* veth_xmit() on the producer side has called netif_tx_stop_queue. The __veth_xdp_flush(rq) will observe rx_notify_masked is false and schedule NAPI.
(2) On the consumer side, the logic for waking the peer TXQ is moved out of veth_xdp_rcv() and placed at the end of the veth_poll() function. This placement is part of fixing the race, as the netif_tx_queue_stopped() check must occur after rx_notify_masked is potentially set to false during NAPI completion. This handles the race where veth_poll() consumes all packets, but haven't finished (rx_notify_masked is still true). The producer veth_xmit() stops the TXQ and __veth_xdp_flush(rq) will observe rx_notify_masked is true, meaning not starting NAPI. Then veth_poll() change rx_notify_masked to false and stops NAPI. Before exiting veth_poll() will observe TXQ is stopped and wake it up.
Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops") Reviewed-by: Toshiaki Makita toshiaki.makita1@gmail.com Signed-off-by: Jesper Dangaard Brouer hawk@kernel.org Link: https://patch.msgid.link/176295323282.307447.14790015927673763094.stgit@fire... Signed-off-by: Jakub Kicinski kuba@kernel.org Stable-dep-of: a14602fcae17 ("veth: reduce XDP no_direct return section to fix race") Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/net/veth.c | 38 ++++++++++++++++++++------------------ 1 file changed, 20 insertions(+), 18 deletions(-)
diff --git a/drivers/net/veth.c b/drivers/net/veth.c index 25b43036cc08b..0f37e056b3dd7 100644 --- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -391,14 +391,12 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev) } /* Restore Eth hdr pulled by dev_forward_skb/eth_type_trans */ __skb_push(skb, ETH_HLEN); - /* Depend on prior success packets started NAPI consumer via - * __veth_xdp_flush(). Cancel TXQ stop if consumer stopped, - * paired with empty check in veth_poll(). - */ netif_tx_stop_queue(txq); - smp_mb__after_atomic(); - if (unlikely(__ptr_ring_empty(&rq->xdp_ring))) - netif_tx_wake_queue(txq); + /* Makes sure NAPI peer consumer runs. Consumer is responsible + * for starting txq again, until then ndo_start_xmit (this + * function) will not be invoked by the netstack again. + */ + __veth_xdp_flush(rq); break; case NET_RX_DROP: /* same as NET_XMIT_DROP */ drop: @@ -900,17 +898,9 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget, struct veth_xdp_tx_bq *bq, struct veth_stats *stats) { - struct veth_priv *priv = netdev_priv(rq->dev); - int queue_idx = rq->xdp_rxq.queue_index; - struct netdev_queue *peer_txq; - struct net_device *peer_dev; int i, done = 0, n_xdpf = 0; void *xdpf[VETH_XDP_BATCH];
- /* NAPI functions as RCU section */ - peer_dev = rcu_dereference_check(priv->peer, rcu_read_lock_bh_held()); - peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL; - for (i = 0; i < budget; i++) { void *ptr = __ptr_ring_consume(&rq->xdp_ring);
@@ -959,9 +949,6 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget, rq->stats.vs.xdp_packets += done; u64_stats_update_end(&rq->stats.syncp);
- if (peer_txq && unlikely(netif_tx_queue_stopped(peer_txq))) - netif_tx_wake_queue(peer_txq); - return done; }
@@ -969,12 +956,20 @@ static int veth_poll(struct napi_struct *napi, int budget) { struct veth_rq *rq = container_of(napi, struct veth_rq, xdp_napi); + struct veth_priv *priv = netdev_priv(rq->dev); + int queue_idx = rq->xdp_rxq.queue_index; + struct netdev_queue *peer_txq; struct veth_stats stats = {}; + struct net_device *peer_dev; struct veth_xdp_tx_bq bq; int done;
bq.count = 0;
+ /* NAPI functions as RCU section */ + peer_dev = rcu_dereference_check(priv->peer, rcu_read_lock_bh_held()); + peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL; + xdp_set_return_frame_no_direct(); done = veth_xdp_rcv(rq, budget, &bq, &stats);
@@ -996,6 +991,13 @@ static int veth_poll(struct napi_struct *napi, int budget) veth_xdp_flush(rq, &bq); xdp_clear_return_frame_no_direct();
+ /* Release backpressure per NAPI poll */ + smp_rmb(); /* Paired with netif_tx_stop_queue set_bit */ + if (peer_txq && netif_tx_queue_stopped(peer_txq)) { + txq_trans_cond_update(peer_txq); + netif_tx_wake_queue(peer_txq); + } + return done; }