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

[Maxime Coquelin]

On 9/18/20 12:46 PM, Vipul Ashri wrote:
> 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!

Thanks Vipul, I plan to pick your patch later today.

So I will assume Andrew still ack the patch since v5 is only fixing
build.

Thanks,
Maxime

> Regards
> Vipul
> 
> -----Original Message-----
> From: Edward Makarov [mailto:makarov at kraftway.ru] 
> Sent: Sunday, 30 August, 2020 3:48
> To: Andrew Rybchenko <arybchenko at solarflare.com>; Vipul Ashri <vipul.ashri at oracle.com>; dev at dpdk.org
> Cc: chenbo.xia at intel.com; zhihong.wang at intel.com; maxime.coquelin at redhat.com; stable at 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: 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.
> 
> 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$
>