Hardware transactional memory (HTM) systems may guarantee that transactions commit without falling back to non-speculative code paths. A transaction that fails to progress may enter a power mode, giving the transaction priority when it conflicts with non-power-mode transactions. If, during execution of a power-mode transaction, another thread attempts, using a non-power-mode transaction, to access a shared resource being accessed by the power-mode transaction, it may be determined whether any actual data conflict occurs between the two transactions. If no data conflict exists, both transactions may continue to completion. If, however, a data conflict does exist, the power-mode transaction may deny the other transaction access to the shared resource. HTM systems may, in some embodiments, ensure that only one power-mode transaction exists at a time. In other embodiments, multiple, concurrent, power-mode transactions may be supported while ensuring that they access disjoint data sets.