[PATCH v3] virtio_ring: Fix KCSAN warnings in virtqueue_enable_cb_delayed_split()

[Chaohai Chen]

KCSAN warns as follow:

[  109.277250] ==================================================================
[  109.283600] BUG: KCSAN: data-race in virtqueue_enable_cb_delayed_split+0x10f/0x170

[  109.295263] race at unknown origin, with read to 0xffff8b2a92ef2042 of 2 bytes by interrupt on cpu 1:
[  109.306934]  virtqueue_enable_cb_delayed_split+0x10f/0x170
[  109.312880]  virtqueue_enable_cb_delayed+0x3b/0x70
[  109.318852]  start_xmit+0x315/0x860 [virtio_net]
[  109.324532]  dev_hard_start_xmit+0x85/0x380
[  109.329993]  sch_direct_xmit+0xd3/0x680
[  109.335360]  __dev_xmit_skb+0x4ee/0xcc0
[  109.340568]  __dev_queue_xmit+0x560/0xe00
[  109.345701]  ip_finish_output2+0x49a/0x9b0
[  109.350743]  __ip_finish_output+0x131/0x250
[  109.355789]  ip_finish_output+0x28/0x180
[  109.360712]  ip_output+0xa0/0x1c0
[  109.365479]  __ip_queue_xmit+0x68d/0x9e0
[  109.370156]  ip_queue_xmit+0x33/0x40
[  109.374783]  __tcp_transmit_skb+0x1703/0x1970
[  109.379467]  __tcp_send_ack.part.0+0x1bb/0x320
...
[  109.499585]  do_idle+0x7a/0xe0
[  109.502979]  cpu_startup_entry+0x25/0x30
[  109.506481]  start_secondary+0x116/0x150
[  109.509930]  common_startup_64+0x13e/0x141

[  109.516626] value changed: 0x0029 -> 0x002a
================================================================================

Add READ_ONCE() to fix the warning.

Signed-off-by: Chaohai Chen <wdhh6@xxxxxxxxxx>
---
 drivers/virtio/virtio_ring.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 335692d41617..ceb026161639 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -1112,8 +1112,9 @@ static bool virtqueue_enable_cb_delayed_split(struct vring_virtqueue *vq)
 			&vring_used_event(&vq->split.vring),
 			cpu_to_virtio16(vq->vq.vdev, vq->last_used_idx + bufs));
 
-	if (unlikely((u16)(virtio16_to_cpu(vq->vq.vdev, vq->split.vring.used->idx)
-					- vq->last_used_idx) > bufs)) {
+	if (unlikely((u16)(virtio16_to_cpu(vq->vq.vdev,
+				READ_ONCE(vq->split.vring.used->idx))
+				- vq->last_used_idx) > bufs)) {
 		END_USE(vq);
 		return false;
 	}
-- 
2.43.0