Subject: Re: [PATCH 2/2] virtio_net: fix race in RX VQ processing

From: Sergei Shtylyov <[email-masked]>
Date: Mon, 08 Jul 2013 16:52:26 +0400

Hello.

On 08-07-2013 13:04, Michael S. Tsirkin wrote:

> virtio net called virtqueue_enable_cq on RX path after napi_complete, so
> with NAPI_STATE_SCHED clear - outside the implicit napi lock.
> This violates the requirement to synchronize virtqueue_enable_cq wrt
> virtqueue_add_buf.  In particular, used event can move backwards,
> causing us to lose interrupts.
> In a debug build, this can trigger panic within START_USE.

> Jason Wang reports that he can trigger the races artificially,
> by adding udelay() in virtqueue_enable_cb() after virtio_mb().

> However, we must call napi_complete to clear NAPI_STATE_SCHED before
> polling the virtqueue for used buffers, otherwise napi_schedule_prep in
> a callback will fail, causing us to lose RX events.

> To fix, call virtqueue_enable_cb_prepare with NAPI_STATE_SCHED
> set (under napi lock), later call virtqueue_poll with
> NAPI_STATE_SCHED clear (outside the lock).

> Reported-by: Jason Wang <[email-masked]>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>   drivers/net/virtio_net.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)

> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 5305bd1..fbdd79a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -622,8 +622,9 @@ again:
>
>   	/* Out of packets? */
>   	if (received < budget) {
> +		unsigned r = virtqueue_enable_cb_prepare(rq->vq);

    Empty line wouldn't hurt here, after declaration.

WBR, Sergei