netdev - Re: [PATCH net-next v4 3/6] virtio_net: Add a lock for the command VQ.

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Message-ID: <CACGkMEsCm3=7FtnsTRx5QJo3ZM0Ko1OEvssWew_tfxm5V=MXvQ@mail.gmail.com>
Date: Thu, 18 Apr 2024 14:42:27 +0800
From: Jason Wang <jasowang@...hat.com>
To: Daniel Jurgens <danielj@...dia.com>
Cc: netdev@...r.kernel.org, mst@...hat.com, xuanzhuo@...ux.alibaba.com, 
	virtualization@...ts.linux.dev, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, jiri@...dia.com
Subject: Re: [PATCH net-next v4 3/6] virtio_net: Add a lock for the command VQ.

On Wed, Apr 17, 2024 at 3:31 AM Daniel Jurgens <danielj@...dia.com> wrote:
>
> The command VQ will no longer be protected by the RTNL lock. Use a
> spinlock to protect the control buffer header and the VQ.
>
> Signed-off-by: Daniel Jurgens <danielj@...dia.com>
> Reviewed-by: Jiri Pirko <jiri@...dia.com>
> ---
>  drivers/net/virtio_net.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 0ee192b45e1e..d02f83a919a7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -282,6 +282,7 @@ struct virtnet_info {
>
>         /* Has control virtqueue */
>         bool has_cvq;
> +       spinlock_t cvq_lock;

Spinlock is instead of mutex which is problematic as there's no
guarantee on when the driver will get a reply. And it became even more
serious after 0d197a147164 ("virtio-net: add cond_resched() to the
command waiting loop").

Any reason we can't use mutex?

Thanks

>
>         /* Host can handle any s/g split between our header and packet data */
>         bool any_header_sg;
> @@ -2529,6 +2530,7 @@ static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
>         /* Caller should know better */
>         BUG_ON(!virtio_has_feature(vi->vdev, VIRTIO_NET_F_CTRL_VQ));
>
> +       guard(spinlock)(&vi->cvq_lock);
>         vi->ctrl->status = ~0;
>         vi->ctrl->hdr.class = class;
>         vi->ctrl->hdr.cmd = cmd;
> @@ -4818,8 +4820,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>             virtio_has_feature(vdev, VIRTIO_F_VERSION_1))
>                 vi->any_header_sg = true;
>
> -       if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ))
> +       if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ)) {
>                 vi->has_cvq = true;
> +               spin_lock_init(&vi->cvq_lock);
> +       }
>
>         if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
>                 mtu = virtio_cread16(vdev,
> --
> 2.34.1
>