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

[Vipul Ashri <vipul.ashri@oracle.com>]

Hi Andrew

I like your suggestion and applied with [v5] net/virtio: fix wrong variable assignment in helper macro
V4 had a extra line typos., V5 is tested compiled and pushed carefully
Thanks!

Regards
Vipul


-----Original Message-----
From: Andrew Rybchenko [mailto:arybchenko@solarflare.com] 
Sent: Saturday, 29 August, 2020 16:53
To: Vipul Ashri <vipul.ashri@oracle.com>; dev@dpdk.org
Cc: chenbo.xia@intel.com; zhihong.wang@intel.com; maxime.coquelin@redhat.com; stable@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in helper macro

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@dpdk.org
>
> Signed-off-by: Vipul Ashri <vipul.ashri@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.