diff --git a/CppCoreGuidelines.md b/CppCoreGuidelines.md index 33e8807..c87f7e7 100644 --- a/CppCoreGuidelines.md +++ b/CppCoreGuidelines.md @@ -13454,7 +13454,7 @@ Example with thread-safe static local variables of C++11. public: My_class() { - // ... + // do this only once } }; @@ -13469,42 +13469,48 @@ Example with thread-safe static local variables of C++11. 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) and [CP.100: Don't use lock-free programming unless you absolutely have to](#Rconc-lockfree), then do it in a conventional pattern. +The uses of double-checked locking pattern that are not in violation of [CP.110: Do not write your own double-checked locking for initialization](#Rconc-double) arise when a non-thread-safe action is both hard and rare, and there exists a fast thread-safe test that can be used to guarantees that the action is not needed, but cannot be used to guarantee the converse. + ##### 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. +The use of volatile does not make the first check thread-safe, see also [CP.200: Use `volatile` only to talk to non-C++ memory](#Rconc-volatile2) - atomic x_init; + mutex action_mutex; + volatile bool action_needed; - if (!x_init.load(memory_order_acquire)) { - lock_guard lck(x_mutex); - if (!x_init.load(memory_order_relaxed)) { - // ... initialize x ... - x_init.store(true, memory_order_release); + if (action_needed) { + std::lock_guard lock(action_mutex); + if (action_needed) { + take_action(); + action_needed = false; } } ##### Example, good -One of the conventional patterns is below. + mutex action_mutex; + atomic action_needed; - 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; + if (action_needed) { + std::lock_guard lock(action_mutex); + if (action_needed) { + take_action(); + action_needed = false; } } -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). +Fine-tuned memory order may be beneficial where acquire load is more efficient than sequentially-consistent load + + mutex action_mutex; + atomic action_needed; + + if (action_needed.load(memory_order_acquire)) { + lock_guard lock(action_mutex); + if (action_needed.load(memory_order_relaxed)) { + take_action(); + action_needed.store(false, memory_order_release); + } + } ##### Enforcement