Add function for full memory barrier into maxscale/atomic.h

This allows safer lock-free reads to be done on lists that never shrink in
size. The main use-case for this is to allow servers to be added to a
service without locking the service each time a new session is created.

Synchronizing the memory before adding new components into a list
guarantees that if a session reads from the list and sees the new list
item, the memory pointed by the item is valid.

include/maxscale/atomic.h

@@ -29,7 +29,42 @@
 
 MXS_BEGIN_DECLS
 
-int atomic_add(int *variable, int value);
+/**
+ * Implementation of an atomic add operation for the GCC environment, or the
+ * X86 processor.  If we are working within GNU C then we can use the GCC
+ * atomic add built in function, which is portable across platforms that
+ * implement GCC.  Otherwise, this function currently supports only X86
+ * architecture (without further development).
+ *
+ * Adds a value to the contents of a location pointed to by the first parameter.
+ * The add operation is atomic and the return value is the value stored in the
+ * location prior to the operation. The number that is added may be signed,
+ * therefore atomic_subtract is merely an atomic add with a negative value.
+ *
+ * @param variable      Pointer the the variable to add to
+ * @param value         Value to be added
+ * @return              The value of variable before the add occurred
+ */
 int atomic_add(int *variable, int value);
 
+/**
+ * @brief Impose a full memory barrier
+ *
+ * A full memory barrier guarantees that all store and load operations complete
+ * before the function is called.
+ *
+ * Currently, only the GNUC __sync_synchronize() is used. C11 introduces
+ * standard functions for atomic memory operations and should be taken into use.
+ *
+ * @see https://www.kernel.org/doc/Documentation/memory-barriers.txt
+ */
+static inline void atomic_synchronize()
+{
+#ifdef __GNUC__
+    __sync_synchronize(); /* Memory barrier. */
+#else
+#error "No GNUC atomics available."
+#endif
+}
+
 MXS_END_DECLS

server/core/atomic.c

@@ -12,7 +12,7 @@
  */
 
 /**
- * @file atomic.c  - Implementation of atomic opertions for the gateway
+ * @file atomic.c  - Implementation of atomic operations for MaxScale
  *
  * @verbatim
  * Revision History
@@ -23,22 +23,6 @@
  * @endverbatim
  */
 
-/**
- * Implementation of an atomic add operation for the GCC environment, or the
- * X86 processor.  If we are working within GNU C then we can use the GCC
- * atomic add built in function, which is portable across platforms that
- * implement GCC.  Otherwise, this function currently supports only X86
- * architecture (without further development).
- *
- * Adds a value to the contents of a location pointed to by the first parameter.
- * The add operation is atomic and the return value is the value stored in the
- * location prior to the operation. The number that is added may be signed,
- * therefore atomic_subtract is merely an atomic add with a negative value.
- *
- * @param variable      Pointer the the variable to add to
- * @param value         Value to be added
- * @return              The value of variable before the add occurred
- */
 int
 atomic_add(int *variable, int value)
 {