netdev - [PATCH net] virtio-net: don't re-enable refill work too early when NAPI is disabled

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Message-ID: <20250429143104.2576553-1-kuba@kernel.org>
Date: Tue, 29 Apr 2025 07:31:04 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org,
	edumazet@...gle.com,
	pabeni@...hat.com,
	andrew+netdev@...n.ch,
	horms@...nel.org,
	Jakub Kicinski <kuba@...nel.org>,
	mst@...hat.com,
	jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com,
	minhquangbui99@...il.com,
	romieu@...zoreil.com,
	kuniyu@...zon.com,
	virtualization@...ts.linux.dev
Subject: [PATCH net] virtio-net: don't re-enable refill work too early when NAPI is disabled

Commit 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
fixed a deadlock between reconfig paths and refill work trying to disable
the same NAPI instance. The refill work can't run in parallel with reconfig
because trying to double-disable a NAPI instance causes a stall under the
instance lock, which the reconfig path needs to re-enable the NAPI and
therefore unblock the stalled thread.

There are two cases where we re-enable refill too early. One is in the
virtnet_set_queues() handler. We call it when installing XDP:

   virtnet_rx_pause_all(vi);
   ...
   virtnet_napi_tx_disable(..);
   ...
   virtnet_set_queues(..);
   ...
   virtnet_rx_resume_all(..);

We want the work to be disabled until we call virtnet_rx_resume_all(),
but virtnet_set_queues() kicks it before NAPIs were re-enabled.

The other case is a more trivial case of mis-ordering in
__virtnet_rx_resume() found by code inspection.

Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
Fixes: 413f0271f396 ("net: protect NAPI enablement with netdev_lock()")
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
CC: mst@...hat.com
CC: jasowang@...hat.com
CC: xuanzhuo@...ux.alibaba.com
CC: eperezma@...hat.com
CC: minhquangbui99@...il.com
CC: romieu@...zoreil.com
CC: kuniyu@...zon.com
CC: virtualization@...ts.linux.dev
---
 drivers/net/virtio_net.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 848fab51dfa1..4c904e176495 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3383,12 +3383,15 @@ static void __virtnet_rx_resume(struct virtnet_info *vi,
 				bool refill)
 {
 	bool running = netif_running(vi->dev);
+	bool schedule_refill = false;
 
 	if (refill && !try_fill_recv(vi, rq, GFP_KERNEL))
-		schedule_delayed_work(&vi->refill, 0);
-
+		schedule_refill = true;
 	if (running)
 		virtnet_napi_enable(rq);
+
+	if (schedule_refill)
+		schedule_delayed_work(&vi->refill, 0);
 }
 
 static void virtnet_rx_resume_all(struct virtnet_info *vi)
@@ -3728,7 +3731,7 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
 succ:
 	vi->curr_queue_pairs = queue_pairs;
 	/* virtnet_open() will refill when device is going to up. */
-	if (dev->flags & IFF_UP)
+	if (dev->flags & IFF_UP && vi->refill_enabled)
 		schedule_delayed_work(&vi->refill, 0);
 
 	return 0;
-- 
2.49.0