os_aio_validate_skip(): Fix a data race

storage/innobase/os/os0file.cc

@@ -1090,21 +1090,14 @@ os_aio_validate_skip()
 /** Try os_aio_validate() every this many times */
 # define OS_AIO_VALIDATE_SKIP	13
 
-	/** The os_aio_validate() call skip counter.
-	Use a signed type because of the race condition below. */
-	static int os_aio_validate_count = OS_AIO_VALIDATE_SKIP;
+	static int os_aio_validate_count;
 
-	/* There is a race condition below, but it does not matter,
-	because this call is only for heuristic purposes. We want to
-	reduce the call frequency of the costly os_aio_validate()
-	check in debug builds. */
-	--os_aio_validate_count;
-
-	if (os_aio_validate_count > 0) {
-		return(true);
+	if (my_atomic_add32_explicit(&os_aio_validate_count, -1,
+				     MY_MEMORY_ORDER_RELAXED)
+	    % OS_AIO_VALIDATE_SKIP) {
+		return true;
 	}
 
-	os_aio_validate_count = OS_AIO_VALIDATE_SKIP;
 	return(os_aio_validate());
 }
 #endif /* UNIV_DEBUG */