Re: [PATCH net v2] virtio-net: fix possible dim status unrecoverable

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2024-03-26 at 14:25 +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.
> 
> Patch has been tested on a VM with 16 NICs, 128 queues per NIC
> (2kq total):
> With dim enabled on all queues, there are many opportunities for
> contention for RTNL lock, and this patch introduces no visible hotspots.
> The dim performance is also stable.
> 
> Fixes: 6208799553a8 ("virtio-net: support rx netdim")
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Acked-by: Jason Wang <jasowang@redhat.com>
> ---
> v1->v2:
>   - Update commit log. No functional changes.
> 
>  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 c22d111..0ebe322 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3563,8 +3563,10 @@ 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;

I'm really scared by this change. VMs are (increasingly) used to run
containers orchestration, which in turns puts a lot of pressure on the
RTNL lock. Any rtnl_trylock+ reschedule may hang for a very long time.
Addressing this kind of issues later becomes _extremely_ painful, see:

https://lore.kernel.org/netdev/20231018154804.420823-1-atenart@kernel.org/

I really think a different solution is needed. What about moving
virtnet_send_command() under protection of a new mutex?

I understand it will complicate future hardening works around cvq, but
really rtnl_trylock()/<spin/retry> is bad for the whole system.

Cheers,

Paolo