[dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in helper macro

[Andrew Rybchenko]

On 8/14/20 12:21 PM, Vipul Ashri wrote:
> Inside Macro ASSIGN_UNLESS_EQUAL(var, val), assignment to var is always
> failing as assignment done using var_ having local scope only.
> This leads to TX packets not going out and found broken due to cleanup
> malfunctioning. This patch fixes the wrong variable assignment.
>
> Fixes: 57f90f894588 ("net/virtio: reuse packed ring functions")
> Cc: stable at dpdk.org
>
> Signed-off-by: Vipul Ashri <vipul.ashri at oracle.com>
> ---
>  drivers/net/virtio/virtqueue.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
> index 105a9c00c..20c95471e 100644
> --- a/drivers/net/virtio/virtqueue.h
> +++ b/drivers/net/virtio/virtqueue.h
> @@ -607,10 +607,8 @@ virtqueue_notify(struct virtqueue *vq)
>  
>  /* avoid write operation when necessary, to lessen cache issues */
>  #define ASSIGN_UNLESS_EQUAL(var, val) do {	\
> -	typeof(var) var_ = (var);		\
> -	typeof(val) val_ = (val);		\
> -	if ((var_) != (val_))			\
> -		(var_) = (val_);		\
> +	if ((var) != (val))			\
> +		(var) = (val);			\

Good catch. As I understand the old code tries to avoid
processing of var and val expressions twice. It looks it
could be kept for val at least. Just keep if condition as in
old code and fix the last line above:
    (var) = val_;
Since var_ and val_are local variables there is no necessity
to enclose it in parenthesis (but does not harm if done).
var_ may be really removed since since resulting code will
use it only once.