netdev - [PATCH Repost to netdev] virtio_net: Fix recursive call to cpus_read

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Date:   Tue, 22 Dec 2020 21:54:21 -0500
From:   Jeff Dike <jdike@...mai.com>
To:     <netdev@...r.kernel.org>
CC:     <jdike@...mai.com>, <virtualization@...ts.linux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, <stable@...r.kernel.org>
Subject: [PATCH Repost to netdev] virtio_net: Fix recursive call to cpus_read_lock()

virtnet_set_channels can recursively call cpus_read_lock if CONFIG_XPS
and CONFIG_HOTPLUG are enabled.

The path is:
    virtnet_set_channels - calls get_online_cpus(), which is a trivial
wrapper around cpus_read_lock()
    netif_set_real_num_tx_queues
    netif_reset_xps_queues_gt
    netif_reset_xps_queues - calls cpus_read_lock()

This call chain and potential deadlock happens when the number of TX
queues is reduced.

This commit the removes netif_set_real_num_[tr]x_queues calls from
inside the get/put_online_cpus section, as they don't require that it
be held.

Signed-off-by: Jeff Dike <jdike@...mai.com>
Acked-by: Jason Wang <jasowang@...hat.com>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Cc: stable@...r.kernel.org
---
 drivers/net/virtio_net.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 052975ea0af4..e02c7e0f1cf9 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2093,14 +2093,16 @@ static int virtnet_set_channels(struct net_device *dev,
 
 	get_online_cpus();
 	err = _virtnet_set_queues(vi, queue_pairs);
-	if (!err) {
-		netif_set_real_num_tx_queues(dev, queue_pairs);
-		netif_set_real_num_rx_queues(dev, queue_pairs);
-
-		virtnet_set_affinity(vi);
+	if (err){
+		put_online_cpus();
+		goto err;
 	}
+	virtnet_set_affinity(vi);
 	put_online_cpus();
 
+	netif_set_real_num_tx_queues(dev, queue_pairs);
+	netif_set_real_num_rx_queues(dev, queue_pairs);
+ err:
 	return err;
 }
 
-- 
2.17.1