Re: C++20 report error at file rte_spinlock.h

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Tue, Dec 13, 2022 at 06:11:06AM +0000, Zhou, Xiangyun wrote:
> Dear dpdk dev,
> 
> I'm using dpdk 21.11 LTS, when compile my program with CPP flag "-std=c++20", the compiler report below errors. After checking file rte_spinlock.h, I think the error report by compiler is valid, there should be a potential issue when using functions rte_spinlock_recursive_lock, rte_spinlock_recursive_unlock and rte_spinlock_recursive_trylock in multi-thread, we could either remove "volatile" definition to ask users to handle the multi-thread issue, or using atomic operatings instead of self-increment and self-decrement.
> 
> 
> /home/dpdk/lib/eal/include/generic/rte_spinlock.h:221:12: error: increment of object of volatile-qualified type 'volatile int' is deprecated [-Werror,-Wdeprecated-volatile]
>         slr->count++;
>                   ^
> /home/dpdk/lib/eal/include/generic/rte_spinlock.h:231:6: error: decrement of object of volatile-qualified type 'volatile int' is deprecated [-Werror,-Wdeprecated-volatile]
>         if (--(slr->count) == 0) {
>             ^
> /home/dpdk/lib/eal/include/generic/rte_spinlock.h:255:12: error: increment of object of volatile-qualified type 'volatile int' is deprecated [-Werror,-Wdeprecated-volatile]
>         slr->count++;
> 

i have work in progress to optionally use standard atomics but in the
meantime the correct thing to do here is to use the gcc builtins that
match the requirements of the c++11 memory model.

the code should be converted to use __atomic_fetch_{add,sub} or
__atomic_{add,sub}_fetch as appropriate.

ty.