Re: [PATCH] usbip: vhci_hcd: slighly simplify code in 'vhci_urb_dequeue()'

[shuah <shuah@kernel.org>]

On 3/21/20 9:29 AM, Christophe JAILLET wrote:
> The allocation of 'unlink' can be moved before a spin_lock.
> This slighly simplifies the error handling if the memory allocation fails,

slightly (spelling nit)

> aligns the code structure with what is done in 'vhci_tx_urb()' and reduces
> potential lock contention.
> 

Are you seeing any problems or is this a potential lock contention?
If you are seeing issues, please share the problem seen.


> Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> ---
>   drivers/usb/usbip/vhci_hcd.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
> index 65850e9c7190..b909a634260c 100644
> --- a/drivers/usb/usbip/vhci_hcd.c
> +++ b/drivers/usb/usbip/vhci_hcd.c
> @@ -905,17 +905,16 @@ static int vhci_urb_dequeue(struct usb_hcd *hcd, struct urb *urb, int status)
>   		/* tcp connection is alive */
>   		struct vhci_unlink *unlink;
>   
> -		spin_lock(&vdev->priv_lock);
> -

This change might simplify the error path, however it could
open a race window with the unlink activity during 
vhci_shutdown_connection() when the connection is being taken
down. It would be safer to hold both locks as soon as the
connection check is done.

>   		/* setup CMD_UNLINK pdu */
>   		unlink = kzalloc(sizeof(struct vhci_unlink), GFP_ATOMIC);
>   		if (!unlink) {
> -			spin_unlock(&vdev->priv_lock);
>   			spin_unlock_irqrestore(&vhci->lock, flags);
>   			usbip_event_add(&vdev->ud, VDEV_EVENT_ERROR_MALLOC);
>   			return -ENOMEM;
>   		}
>   
> +		spin_lock(&vdev->priv_lock);
> +
>   		unlink->seqnum = atomic_inc_return(&vhci_hcd->seqnum);
>   		if (unlink->seqnum = 0xffff)
>   			pr_info("seqnum max\n");
> 

thanks,
-- Shuah