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

[Edward Makarov <makarov@kraftway.ru>]

On 8/29/20 2:22 PM, Andrew Rybchenko wrote:
> 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.

I think there is a solution to avoid multiple evaluations of parameters:

// var is definitely an lvalue, so its address can definitely be taken
#define ASSIGN_UNLESS_EQUAL(var, val) do {  \
	typeof(var) *const var_ = &(var);   \
	typeof(val)  const val_ = (val);    \
	if (*var_ != val_)                  \
		*var_ = val_;               \
} while (0)

The solution relies on the compiler optimizer. Here is the comparison of
what the variants are compiled into:

https://godbolt.org/z/nnvq5q