diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md
index a3dd5ce..68099c0 100644
--- a/CppCoreGuidelines.md
+++ b/CppCoreGuidelines.md
@@ -12503,7 +12503,8 @@ Lock-free programming rule summary:
* how/when to use atomics
* avoid starvation
* use a lock free data structure rather than hand-crafting specific lock-free access
-* [CP.110: Use a conventional pattern for double-checked locking](#Rconc-double)
+* [CP.110: Do not write your own double-checked locking for initialization](#Rconc-double-init)
+* [CP.111: Use a conventional pattern if you really need double-checked locking](#Rconc-double-pattern)
* how/when to compare and swap
@@ -12580,14 +12581,59 @@ Become an expert before shipping lock-free code for others to use.
* Damian Dechev, Peter Pirkelbauer, Nicolas Rouquette, and Bjarne Stroustrup: Semantically Enhanced Containers for Concurrent Real-Time Systems. Proc. 16th Annual IEEE International Conference and Workshop on the Engineering of Computer Based Systems (IEEE ECBS). April 2009.
-### CP.110: Use a conventional pattern for double-checked locking
+### CP.110: Do not write your own double-checked locking for initialization
##### Reason
-Double-checked locking is easy to mess up.
+Since C++11, static local variables are now initialized in a thread-safe way. When combined with the RAII pattern, static local variables can replace the need for writing your own double-checked locking for initialization. std::call_once can also achieve the same purpose. Use either static local variables of C++11 or std::call_once instead of writing your own double-checked locking for initialization.
##### Example
+Example with std::call_once.
+
+ void f()
+ {
+ static std::once_flag my_once_flag;
+ std::call_once(my_once_flag, []()
+ {
+ // do this only once
+ });
+ // ...
+ }
+
+Example with thread-safe static local variables of C++11.
+
+ void f()
+ {
+ // Assuming the compiler is compliant with C++11
+ static My_class my_object; // Constructor called only once
+ // ...
+ }
+
+ class My_class
+ {
+ public:
+ My_class()
+ {
+ // ...
+ }
+ };
+
+##### Enforcement
+
+??? Is it possible to detect the idiom?
+
+
+### CP.111: Use a conventional pattern if you really need double-checked locking
+
+##### Reason
+
+Double-checked locking is easy to mess up. If you really need to write your own double-checked locking, in spite of the rules [CP.110: Do not write your own double-checked locking for initialization](#Rconc-double-init) and [CP.100: Don't use lock-free programming unless you absolutely have to](#Rconc-lockfree), then do it in a conventional pattern.
+
+##### Example, bad
+
+Even if the following example works correctly on most hardware platforms, it is not guaranteed to work by the C++ standard. The x_init.load(memory_order_relaxed) call may see a value from outside of the lock guard.
+
atomic x_init;
if (!x_init.load(memory_order_acquire)) {
@@ -12598,8 +12644,28 @@ Double-checked locking is easy to mess up.
}
}
- // ... use x ...
+##### Example, good
+One of the conventional patterns is below.
+
+ std::atomic state;
+
+ // If state == SOME_ACTION_NEEDED maybe an action is needed, maybe not, we need to
+ // check again in a lock. However, if state != SOME_ACTION_NEEDED, then we can be
+ // sure that an action is not needed. This is the basic assumption of double-checked
+ // locking.
+
+ if (state == SOME_ACTION_NEEDED)
+ {
+ std::lock_guard lock(mutex);
+ if (state == SOME_ACTION_NEEDED)
+ {
+ // do something
+ state = NO_ACTION_NEEDED;
+ }
+ }
+
+In the example above (state == SOME_ACTION_NEEDED) could be any condition. It doesn't necessarily needs to be equality comparison. For example, it could as well be (size > MIN_SIZE_TO_TAKE_ACTION).
##### Enforcement