Re: [PATCH v2] virtio_ring: tag event_triggered as racy for KCSAN

[Marco Elver]

On Thu, 12 Sept 2024 at 17:02, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> Setting event_triggered from the interrupt handler
> is fundamentally racy. There are races of 2 types:
> 1. vq processing can read false value while interrupt
>    triggered and set it to true.
>    result will be a bit of extra work when disabling cbs, no big deal.
>
> 1. vq processing can set false value then interrupt
>    immediately sets true value
>    since interrupt then triggers a callback which will
>    process buffers, this is also not an issue.
>
> However, looks like KCSAN can not figure all this out, and warns about
> the race between the write and the read.  Tag the access data_racy for
> now.  We should probably look at ways to make this more
> straight-forwardly correct.
>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Reported-by: syzbot+8a02104389c2e0ef5049@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Probably more conservative than the __data_racy hammer:

Acked-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  drivers/virtio/virtio_ring.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index be7309b1e860..98374ed7c577 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -2588,7 +2588,7 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
>
>         /* Just a hint for performance: so it's ok that this can be racy! */
>         if (vq->event)
> -               vq->event_triggered = true;
> +               data_race(vq->event_triggered = true);
>
>         pr_debug("virtqueue callback for %p (%p)\n", vq, vq->vq.callback);
>         if (vq->vq.callback)
> --
> MST
>