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://godbolt.org/z/nnvq5q
