Re: [PATCH] qemu/atomic.h: add #ifdef guards for stdatomic.h

["Alex Bennée" <alex.bennee@linaro.org>]

Peter Maydell <peter.maydell@linaro.org> writes:

> On Thu, 26 Mar 2020 at 17:01, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> Deep inside the FreeBSD netmap headers we end up including stdatomic.h
>> which clashes with qemu's atomic functions which are modelled along
>> the C11 standard. To avoid a massive rename lets just ifdef around the
>> problem.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>  include/qemu/atomic.h | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
>> index f9cd24c8994..ff72db51154 100644
>> --- a/include/qemu/atomic.h
>> +++ b/include/qemu/atomic.h
>> @@ -208,11 +208,14 @@
>>  /* Provide shorter names for GCC atomic builtins, return old value */
>>  #define atomic_fetch_inc(ptr)  __atomic_fetch_add(ptr, 1, __ATOMIC_SEQ_CST)
>>  #define atomic_fetch_dec(ptr)  __atomic_fetch_sub(ptr, 1, __ATOMIC_SEQ_CST)
>> +
>> +#ifndef atomic_fetch_add
>>  #define atomic_fetch_add(ptr, n) __atomic_fetch_add(ptr, n, __ATOMIC_SEQ_CST)
>>  #define atomic_fetch_sub(ptr, n) __atomic_fetch_sub(ptr, n, __ATOMIC_SEQ_CST)
>>  #define atomic_fetch_and(ptr, n) __atomic_fetch_and(ptr, n, __ATOMIC_SEQ_CST)
>>  #define atomic_fetch_or(ptr, n)  __atomic_fetch_or(ptr, n, __ATOMIC_SEQ_CST)
>>  #define atomic_fetch_xor(ptr, n) __atomic_fetch_xor(ptr, n, __ATOMIC_SEQ_CST)
>> +#endif
>
> This will work around FreeBSD's current implementation in particular,
> but I don't think there's anything in the C11 spec that mandates that
> atomic_fetch_add() and friends have to be macros and not simply
> functions...

Sure there are two alternative options:

 - Move to using stdatomic headers - on Linux they seem to be C++ only
 - Rename all out atomic functions - seems a bit of a big patch for rc releases

I suspect we should look at option two for 5.1

-- 
Alex Bennée