All callers of QContiguousCache<T>::allocateData() followed the call with a storeRelaxed(1) to the ref member. So we can just drag that into the function itself. Next, it's UB to storeRelaxed() into a default-constructed std::atomic (and, therefore, into a QBasicAtomicInt), because until C++17 (inclusive) you're supposed to use std::atomic_init to assign the first (and only the first) value to a default-constructed std::atomic. QBasicAtomic doesn't have API for that, so you can never assign anything to a default-constructed QBasicAtomic. To fix, use placement new to be able to create a QBasicAtomic directly with an initial value (replacing QBasicAtomic with QAtomic wouldn't help here, either, since a malloc doesn't run ctors). A proper fix has to wait until we can depend on C++20's atomic_ref, which decouples the underlying type from the atomic operations performed on it, letting us depend on malloc's zero-initialization of an int member properly initializing it even for a following atomic operation on it. Task-number: QTBUG-137465 Pick-to: 6.10 6.9 6.8 6.5 Change-Id: Ic22d0766bcffb967a86c8ec28b63ee480aebd4a0 Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>