Hi Edward / Andrew

I like your suggestions 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: Edward Makarov [mailto:maka...@kraftway.ru] 
Sent: Sunday, 30 August, 2020 3:48
To: Andrew Rybchenko <arybche...@solarflare.com>; Vipul Ashri 
<vipul.as...@oracle.com>; dev@dpdk.org
Cc: chenbo....@intel.com; zhihong.w...@intel.com; maxime.coque...@redhat.com; 
sta...@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] net/virtio: fix wrong variable assignment in 
helper macro


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: sta...@dpdk.org
>>
>> Signed-off-by: Vipul Ashri <vipul.as...@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://urldefense.com/v3/__https://godbolt.org/z/nnvq5q__;!!GqivPVa7Brio!MJZCNAgcCHB5A_T1mI2aA-2F9wvqh_WOfzkeN0IbDnsZSlNIPWqDF0b4YDmTc19HcA$

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