I recently diagnosed the root cause of a concurrency bug, CR6822370, and thought it sufficiently interesting to share the details. (CR 6822370 actually represents a cluster of bugs that are now thought to be related by a common underlying issue). Briefly, we have a lost wakeup bug in the native C++ Parker::park() platform-specific infrastructure code that implements java.util.concurrent.LockSupport.park(). The lost wakeup arises from a race that itself arises because of architectural reordering that in turn occurs because of missing memory barrier instructions. The lost wakeup may manifest as various 'hangs' or instances of progress failure.