Subject: Re: [PATCH net-next 1/3] virtio-net: fix possible dim status unrecoverable

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 16 Jan 2024 08:15:10 -0500

On Tue, Jan 16, 2024 at 09:11:31PM +0800, Heng Qi wrote:
> When the dim worker is scheduled, if it fails to acquire the lock,
> dim may not be able to return to the working state later.
> 
> For example, the following single queue scenario:
>   1. The dim worker of rxq0 is scheduled, and the dim status is
>      changed to DIM_APPLY_NEW_PROFILE;
>   2. The ethtool command is holding rtnl lock;
>   3. Since the rtnl lock is already held, virtnet_rx_dim_work fails
>      to acquire the lock and exits;
> 
> Then, even if net_dim is invoked again, it cannot work because the
> state is not restored to DIM_START_MEASURE.
> 
> Fixes: 6208799553a8 ("virtio-net: support rx netdim")
> Signed-off-by: Heng Qi <[email-masked]>
> ---
> Belongs to the net branch.
> 
>  drivers/net/virtio_net.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index d7ce4a1..f6ac3e7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3524,8 +3524,10 @@ static void virtnet_rx_dim_work(struct work_struct *work)
>  	struct dim_cq_moder update_moder;
>  	int i, qnum, err;
>  
> -	if (!rtnl_trylock())
> +	if (!rtnl_trylock()) {
> +		schedule_work(&dim->work);
>  		return;
> +	}
>  
>  	/* Each rxq's work is queued by "net_dim()->schedule_work()"
>  	 * in response to NAPI traffic changes. Note that dim->profile_ix

OK but this means that in cleanup it is not sufficient to flush
dim work - it can requeue itself.

> -- 
> 1.8.3.1