RE: rte_atomic API compatibility & standard atomics

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Tyler Retzlaff [mailto:roretzla@linux.microsoft.com]
> Sent: Monday, 27 March 2023 21.39
> 
> Hi folks,
> 
> I don't think we discussed it specifically but what is the expectation
> in relation to converting to standard atomics and compatibility of the
> legacy rte_atomic APIs?
> 
> We can't really convert the inline function implementations of the
> rte_atomic APIs because doing so would break compatibility. This is
> because if the implementation uses standard atomics APIs then we are
> required to pass _Atomic types to the generic atomic intrinsics.
> 
> We can choose to just leave the rte_atomic API implementations as they
> are using the GCC builtins and i'm fine with that, but I do need some
> help with what to do with msvc then since it doesn't have those
> builtins.
> 
> The options seem to be as follows.
> 
> 1.
> Just cast the non-atomic types in the rte_atomic APIs implementation
> to _Atomic which may work but i'm pretty sure is undefined behavior
> since
> you can't qualify a non _Atomic type to suddenly be _Atomic.
> 
> 2.
> We could conditionally compile (hide) the legacy rte_atomic APIs when
> msvc is in use, this seems not bad since there technically aren't any
> Windows/MSVC consumers, but if someone wanted to port an existing
> application they would have to adapt the code to avoid use of
> rte_atomic.
> 
> For now I think the safest option is to go with 2 since it doesn't
> impose any compatibility risk and conditional compilation only exists
> until we deprecate and remove the old rte_atomic APIs.
> 
> Are there any other options i'm missing here?
> 
> Thanks

As a variant of your second option, you could make most of the legacy rte_atomic APIs available to MSVC by changing the atomic counter types from volatile to _Atomic. Then only the atomic cmpset() and exchange() functions are unavailable for the application. E.g. for the 32 bit atomic counter type:

typedef struct {
-	volatile int32_t cnt; /**< An internal counter value. */
+	_Atomic int32_t cnt; /**< An internal counter value. */
} rte_atomic32_t;