arch,lib/seqlock: implement seqlock with C++ atomic if compiled with C++

because some functions declared by <stdatomic.h> share the same names
with those declared by <atomic>, for instance `kill_dependency()` is
defined as a macro by <stdatomic.h>, while it is defined as a template
function in <atomic>.

this renders it impossible to compile an ioengine written in C++ if
its source file includes both <atomic> and <fio.h>. the latter includes
<stdatomic.h> via arch/arch.h. the compile error would look like:

In file included from ../src/test/fio/fio_ceph_objectstore.cc:26:
In file included from src/fio/fio.h:18:
In file included from src/fio/thread_options.h:8:
In file included from src/fio/gettime.h:7:
src/fio/lib/seqlock.h:21:9: error: expected ')'
                seq = atomic_load_acquire(&s->sequence);
                      ^
src/fio/arch/../lib/../arch/arch.h:47:32: note: expanded from macro 'atomic_load_acquire'
        atomic_load_explicit((_Atomic typeof(*(p)) *)(p),       \
                                      ^
src/fio/lib/seqlock.h:21:9: note: to match this '('

to address this issue, instead of using the functions in <stdatomic.h> to
implement seqlock, use the primitives offered by C++ standard library
if the source code is compiled using a C++ compiler.

Signed-off-by: Kefu Chai <[email protected]>

arch/arch.h

@@ -1,7 +1,11 @@
 #ifndef ARCH_H
 #define ARCH_H
 
+#ifdef __cplusplus
+#include <atomic>
+#else
 #include <stdatomic.h>
+#endif
 
 #include "../lib/types.h"
 
@@ -36,6 +40,21 @@ extern unsigned long arch_flags;
 
 #define ARCH_CPU_CLOCK_WRAPS
 
+#ifdef __cplusplus
+#define atomic_add(p, v)						\
+	std::atomic_fetch_add(p, (v))
+#define atomic_sub(p, v)						\
+	std::atomic_fetch_sub(p, (v))
+#define atomic_load_relaxed(p)					\
+	std::atomic_load_explicit(p,				\
+			     std::memory_order_relaxed)
+#define atomic_load_acquire(p)					\
+	std::atomic_load_explicit(p,				\
+			     std::memory_order_acquire)
+#define atomic_store_release(p, v)				\
+	std::atomic_store_explicit(p, (v),			\
+			     std::memory_order_release)
+#else
 #define atomic_add(p, v)					\
 	atomic_fetch_add((_Atomic typeof(*(p)) *)(p), v)
 #define atomic_sub(p, v)					\
@@ -49,6 +68,7 @@ extern unsigned long arch_flags;
 #define atomic_store_release(p, v)				\
 	atomic_store_explicit((_Atomic typeof(*(p)) *)(p), (v),	\
 			      memory_order_release)
+#endif
 
 /* IWYU pragma: begin_exports */
 #if defined(__i386__)

lib/seqlock.h

@@ -5,7 +5,11 @@
 #include "../arch/arch.h"
 
 struct seqlock {
+#ifdef __cplusplus
+	std::atomic<unsigned int> sequence;
+#else
 	volatile unsigned int sequence;
+#endif
 };
 
 static inline void seqlock_init(struct seqlock *s)