linux-kernel - Re: [PATCH v3 1/2] virtio_console: free unused buffers with port delete

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Date:   Sat, 10 Aug 2019 14:18:48 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     amit@...nel.org, arnd@...db.de, gregkh@...uxfoundation.org,
        virtualization@...ts.linux-foundation.org, jasowang@...hat.com,
        linux-kernel@...r.kernel.org, xiaohli@...hat.com
Subject: Re: [PATCH v3 1/2] virtio_console: free unused buffers with port
 delete

On Fri, Aug 09, 2019 at 12:18:46PM +0530, Pankaj Gupta wrote:
> The commit a7a69ec0d8e4 ("virtio_console: free buffers after reset")
> deferred detaching of unused buffer to virtio device unplug time.
> This causes unplug/replug of single port in virtio device with an
> error "Error allocating inbufs\n". As we don't free the unused buffers
> attached with the port. Re-plug the same port tries to allocate new
> buffers in virtqueue and results in this error if queue is full.

So why not reuse the buffers that are already there in this case?
Seems quite possible.

> This patch removes the unused buffers in vq's when we unplug the port.
> This is the best we can do as we cannot call device_reset because virtio
> device is still active.
> 
> Reported-by: Xiaohui Li <xiaohli@...hat.com>
> Fixes: a7a69ec0d8e4 ("virtio_console: free buffers after reset")
> Cc: stable@...r.kernel.org
> Signed-off-by: Pankaj Gupta <pagupta@...hat.com>

This is really a revert of a7a69ec0d8e4, just tagged confusingly.

And the original is also supposed to be a bugfix.
So how will the original bug be fixed?

"this is the best we can do" is rarely the case.

I am not necessarily against the revert. But if we go that way then what
we need to do is specify the behaviour in the spec, since strict spec
compliance is exactly what the original patch was addressing.

In particular, we'd document that console has a special property that
when port is detached virtqueue is considered stopped, device must not
use any buffers, and it is legal to take buffers out of the device.



> ---
>  drivers/char/virtio_console.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 7270e7b69262..e8be82f1bae9 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1494,15 +1494,25 @@ static void remove_port(struct kref *kref)
>  	kfree(port);
>  }
>  
> +static void remove_unused_bufs(struct virtqueue *vq)
> +{
> +	struct port_buffer *buf;
> +
> +	while ((buf = virtqueue_detach_unused_buf(vq)))
> +		free_buf(buf, true);
> +}
> +
>  static void remove_port_data(struct port *port)
>  {
>  	spin_lock_irq(&port->inbuf_lock);
>  	/* Remove unused data this port might have received. */
>  	discard_port_data(port);
> +	remove_unused_bufs(port->in_vq);
>  	spin_unlock_irq(&port->inbuf_lock);
>  
>  	spin_lock_irq(&port->outvq_lock);
>  	reclaim_consumed_buffers(port);
> +	remove_unused_bufs(port->out_vq);
>  	spin_unlock_irq(&port->outvq_lock);
>  }
>  
> @@ -1938,11 +1948,9 @@ static void remove_vqs(struct ports_device *portdev)
>  	struct virtqueue *vq;
>  
>  	virtio_device_for_each_vq(portdev->vdev, vq) {
> -		struct port_buffer *buf;
>  
>  		flush_bufs(vq, true);
> -		while ((buf = virtqueue_detach_unused_buf(vq)))
> -			free_buf(buf, true);
> +		remove_unused_bufs(vq);
>  	}
>  	portdev->vdev->config->del_vqs(portdev->vdev);
>  	kfree(portdev->in_vqs);
> -- 
> 2.21.0