Adding one shared reference doesn't work because the number of shared references can be negative (if non-owning threads end up removing more references than the

Adding one shared reference doesn't work because the number of shared references can be negative (if non-owning threads end up removing more references than they add: think of the owner storing objects in a collection and a bunch of consumers that pick them and replace them with None). The meaning of the extra bit is "the object doesn't have an owning thread anymore, and the local refcount will always be zero; and the shared refcount cannot go negative anymore so a zero really means the object can be freed". A second bit is used if a non-owning thread notices local+shared=0. In that case the object is placed on a list that the owning thread will go through every now and then. For all objects in the list, the owning thread transfers the local references to the shared refcount, disowns the object (by setting the first special bit). Then, according to the rules for the first social bit, if the resulting shared refcount is still zero the object can be freed.