1. A system, comprising: one or more processors; and a memory coupled to the one or more processors, wherein the memory stores program instructions executable by the one or more processors to implement a memory manager configured to: coordinate memory access requests specifying data locations within the memory, wherein said coordinating comprises: recording, within a collaborator record of a first shared data object in the memory, identifications of a set of two or more transactions that have each requested synchronization on the first shared data object; in response to a commit request from a given transaction of the set, determining whether to commit or abort the given transaction based at least in part on transactional states of one or more other transactions of the set, wherein said determining comprises examining the collaborator record to identify the one or more other transactions of the set; and committing or aborting the given transaction according to said determining.