qreadwritelock.cpp

class QReadWriteLockPrivate;
typedef __uintcap_t quintptr;

namespace {
enum {
    StateMask = 0x3,
    StateLockedForRead = 0x1,
    StateLockedForWrite = 0x2,
};
const auto dummyLockedForRead = reinterpret_cast<QReadWriteLockPrivate *>(quintptr(StateLockedForRead));
const auto dummyLockedForWrite = reinterpret_cast<QReadWriteLockPrivate *>(quintptr(StateLockedForWrite));
}

bool testAndSetAcquire(void* expected, void* newValue);
void lockSlowPath();

void lock() {
  // This previously stored a $ddc derived value instead of a null-derived one
  if (testAndSetAcquire(nullptr, dummyLockedForRead))
    return;
  lockSlowPath();
}