Implementation of passively synchronized RMA for shared memory
machines

------------------------------------------------------------------------

Base Assumptions

* All of the local windows are located in shared memory.

* Only basic datatypes are supported for the target.

------------------------------------------------------------------------

General Notes


------------------------------------------------------------------------

Data Structures

* MPID_Win

  * struct MPIR_Win

  * lwin_rwmutexes[np]

  * region_mutexes[nregions]

------------------------------------------------------------------------

MPID_Win_lock

* if MPI_MODE_NOCHECK is not set

  * if lock_type is MPI_LOCK_SHARED, then acquire proc_rwmutexes[rank]
    as a reader, otherwise acquire it as a writer

    NOTE: the read-write mutex should be implemented fairly so that
    writers are not starved by continual overlapping lock requests by
    readers.

* set process local state to indicate whether or not the rw-mutex is held

* set process local state to indicate if the lock is shared or exclusive

------------------------------------------------------------------------

MPID_Win_unlock

* release proc_rwmutex, if it is being held

------------------------------------------------------------------------

MPID_Accumulate

NOTE: When the lock is shared, we can achieve more parallelism by
dividing the local window into regions.  Each region would have a
separate mutex to guarantee that all data elements with that region
were processed atomically.  Ideally, typereps would be optimized such
that a region mutex would never be acquired more than once per
accumulate operation.

NOTE: For machines where intrinsic types must be aligned on boundaries
of that types size, we can ensure that a type does not cross a region
boundary by aligning the region boundaries at addresses divisible by
the size of the largest type and forcing all regions to contain at
least as many bytes as the largest type.  For all other machines,
extra logic will be required to hold multiple mutexes when a type
crosses a region boundary.

NOTE: When the lock is exclusive, slicing up the local window and
optimizing the typereps to increase region locality is unnecessary.
