Testcase: ./tst_qreadwritelock countingTest WARNING: ThreadSanitizer: data race (pid=356186) Read of size 1 at 0x7294000000f8 by thread T12: #0 contendedTryLockForRead qtbase/src/corelib/thread/qreadwritelock.cpp:230 (libQt6Core_tsan.so.6+0x6c3743) #1 QReadWriteLock::tryLockForRead(QDeadlineTimer) qtbase/src/corelib/thread/qreadwritelock.cpp:190 (libQt6Core_tsan.so.6+0x6c347b) #2 QReadWriteLock::lockForRead() qtbase/src/corelib/thread/qreadwritelock.h:68 (tst_qreadwritelock+0xb0f0) #3 ReadLockCountThread::run() qtbase/tests/auto/corelib/thread/qreadwritelock/tst_qreadwritelock.cpp:597 (tst_qreadwritelock+0xc506) Previous write of size 8 at 0x7294000000f8 by thread T2: #0 operator new[](unsigned long, std::align_val_t) <null> (libtsan.so.2+0xa78eb) #1 allocate qtbase/src/corelib/tools/qfreelist_p.h:135 (libQt6Core_tsan.so.6+0x6c6079) #2 next qtbase/src/corelib/tools/qfreelist_p.h:212 (libQt6Core_tsan.so.6+0x6c5915) #3 QReadWriteLockPrivate::allocate() qtbase/src/corelib/thread/qreadwritelock.cpp:564 (libQt6Core_tsan.so.6+0x6c5354) #4 contendedTryLockForRead qtbase/src/corelib/thread/qreadwritelock.cpp:218 (libQt6Core_tsan.so.6+0x6c364f) The loadRelaxed() at the beginning of tryLockForRead/tryLockForWrite isn't enough to bring us the non-atomic write of the recursive bool. Same issue with the std::mutex itself. Pick-to: 6.9 6.8 6.5 Change-Id: I6f5e371cf94292b643cb36041d1406b19d22cdbe Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>