Add atomic store and load operations

Added abstractions for storing and loading 32-bit and 64-bit values
atomically. The functions currently use the GCC __atomic builtin atomics.

include/maxscale/atomic.h

@@ -36,6 +36,32 @@ int      atomic_add(int *variable, int value);
 int64_t  atomic_add_int64(int64_t *variable, int64_t value);
 uint64_t atomic_add_uint64(uint64_t *variable, int64_t value);
 
+/**
+ * Implementation of an atomic load operation for the GCC environment.
+ *
+ * Loads a value from the contents of a location pointed to by the first parameter.
+ * The load operation is atomic and it uses the strongest memory ordering.
+ *
+ * @param variable      Pointer the the variable to load from
+ * @return The stored value
+ */
+int atomic_read(int *variable);
+int64_t atomic_read_int64(int64_t *variable);
+uint64_t atomic_read_uint64(uint64_t *variable);
+
+/**
+ * Implementation of an atomic store operation for the GCC environment.
+ *
+ * Stores a value to the contents of a location pointed to by the first parameter.
+ * The store operation is atomic and it uses the strongest memory ordering.
+ *
+ * @param variable      Pointer the the variable to store to
+ * @param value         Value to be stored
+ */
+void atomic_write(int *variable, int value);
+void atomic_write_int64(int64_t *variable, int64_t value);
+void atomic_write_uint64(uint64_t *variable, uint64_t value);
+
 /**
  * @brief Impose a full memory barrier
  *

server/core/atomic.cc

@@ -31,3 +31,33 @@ uint64_t atomic_add_uint64(uint64_t *variable, int64_t value)
 {
     return __sync_fetch_and_add(variable, value);
 }
+
+int atomic_read(int *variable)
+{
+    return __atomic_load_n(variable, __ATOMIC_SEQ_CST);
+}
+
+int64_t atomic_read_int64(int64_t *variable)
+{
+    return __atomic_load_n(variable, __ATOMIC_SEQ_CST);
+}
+
+uint64_t atomic_read_uint64(uint64_t *variable)
+{
+    return __atomic_load_n(variable, __ATOMIC_SEQ_CST);
+}
+
+void atomic_write(int *variable, int value)
+{
+    return __atomic_store_n(variable, value, __ATOMIC_SEQ_CST);
+}
+
+void atomic_write_int64(int64_t *variable, int64_t value)
+{
+    return __atomic_store_n(variable, value, __ATOMIC_SEQ_CST);
+}
+
+void atomic_write_uint64(uint64_t *variable, uint64_t value)
+{
+    return __atomic_store_n(variable, value, __ATOMIC_SEQ_CST);
+}