Re: [PATCH v2] virtio_ring: Fix data races in virtqueue_enable_cb_delayed_split()

["Michael S. Tsirkin" <mst@redhat.com>]

here's how I would put it:

KCSAN reports .....
this is because ....


This is not a real issue, because ....
To disable the KCSAN warning, use READ_ONCE to read vq->split.vring.used->idx.




On Fri, Mar 06, 2026 at 10:24:48AM +0800, Chaohai Chen wrote:
> Add READ_ONCE when fetching vq->split.vring.used->idx
> without synchronization.

it's with synchronization, of course

> KCSAN detected

detected -> incorrectly reported

> data races when accessing the split virtqueue's
> used ring, which is shared memory concurrently accessed by both the CPU
> and the virtio device (hypervisor).
> 
> Example KCSAN report:
> 
> [  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
> ================================================================================
> 
> Signed-off-by: Chaohai Chen <wdhh6@aliyun.com>



I'd like to see code size before/after compared, and included in
the log. Does this make the compiler's work much harder?

> ---
>  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)) {

leave "-" on the previous line and indent the continuation more please.
This way one can easily see there's a continuation by looking
at both the 1st and the second lines.

>  		END_USE(vq);
>  		return false;
>  	}
> -- 
> 2.43.7