Concurrent threads may be synchronized at the level of the memory words they access rather than at the level of the lock that protects the execution of critical sections. Each lock may be associated with an array of flags and each flag may indicate ownership of certain memory words. A pessimistic thread may set flags corresponding to memory words it is accessing in the critical section, while an optimistic thread may read the corresponding flag before any memory access to ensure that the flag is not set and that therefore the associated memory word is not being accessed by the other thread. Thus, optimistic threads that do not have conflicts with the pessimistic thread may not have to wait for the pessimistic thread to release the lock before proceeding.