1. Use CV_SINGLETON_LAZY_INIT macro, without creation of global objects (you can't guarantee initialization order of them - so Mutex g_matAllocatorMutex; doesn't work because it may not be initialized when you want to use them). See "static initialization order fiasco" for more info.
2. Don't change CV_SINGLETON_LAZY_INIT macro. It is well known pattern: https://en.wikipedia.org/wiki/Double-checked_locking

@alalek, let me assign it to you

@grundman Patch is rebased and updated (race condition in Mat::getDefaultAllocator() is resolved, other changes were removed).

👍