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: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$
>
