Below is the current design for passive target RMA on top of CH3.

We assume that there is some asychronous agent (thread) that
periodically pokes the progress engine, i.e., periodically calls
MPID_Progress_test(). We need to do a general poke of the progress
engine because we don't know whether there will be passive target RMA
or not and there may be other communication going on. 

This thread is created only when MPI_Win_create is called and the user
did not pass an info object with the key "no_locks" set to "true". (As
an aside, I wish this was an assert instead of an info. An assert can
be easily passed, whereas users are not likely to go through the
trouble of creating an info object to say no_locks, even if they only
plan to use fence.)

Assuming that such a thread exists, passive target RMA is implemented
in the CH3 channel as follows:

On the source side, MPI_Win_lock and all the RMA operations after it
are simply queued until MPI_Win_unlock is called (similar to what we
do with fence and start/complete). At MPI_Win_unlock, a "lock" packet
is sent over to the target, containing the lock type and the rank of
the source. The source then waits for a "lock granted" reply from the
target. (Singleton puts/gets are handled differently. See
Optimization for Single Puts/Gets/Accs below.) 

When the target receives a lock packet, it examines the current lock
information for that window (described later below) and either grants
the lock by sending a "lock granted" packet to the source or just
queues up the lock request. 

When the source receives a "lock granted" reply, it performs the RMA
operations exactly as in the fence or start/complete case. The last
RMA operation also releases the lock on the window at the target.
No separate unlock packet needs to be sent.

Note that RMA requests that the target receives (put, get, acc) are
always satisfied, because they won't be sent in the first place unless
the source has been authorized to send them.

If none of the RMA operations is a get, the target must send an
acknowledgement to the source when the last RMA operation has
completed. If any one of the operations is a get, we reorder the
operations and perform the get last. In this case, since the source
must wait to receive the data, the acknowledgement is not needed
assuming that data transfer is ordered. If data transfer is not
ordered, an acknowledgement is needed even if the last operation is a
get.

The MPI_Win object needs the following lock info:
   int current_lock_type;  /* no_lock, shared, exclusive */
   int shared_lock_ref_cnt;  /* count of active shared locks */
   a queue of unsatisfied lock requests;

When a lock request arrives at the target, it looks at the incoming
and existing lock types and takes the following action:

Incoming           Existing             Action
--------           --------             ------
Shared             Exclusive            Queue it
Shared             NoLock/Shared        Grant it
Exclusive          NoLock               Grant it
Exclusive          Exclusive/Shared     Queue it

No change needs to be made to the existing code in the progress engine
for handling put/get/accumulate, except that when the last RMA
operation from a source is completed, grant the next queued lock
request if there is one and change the lock_type if necessary. This
can be done even if the sync model is fence or post/start because in
that case there will be no lock request to grant. Therefore we don't
need to know whether the current synch model is lock/unlock or not.


Optimization for Single Puts, Gets, Accs
----------------------------------------
For the case where the lock/unlock is for a single short
put/accumulate or get, we can send over the put data (or get info)
along with the lock pkt. If the lock needs to be queued, it will be
queued with this data or info. When the lock is granted, no "lock
granted" reply needs to be sent. Instead the put data is simply copied
or the get data is sent over. Except in the case of get operations,
Win_unlock must block until it receives an acknowledgement from the
target that the RMA operation has completed (for both shared and
exclusive locks). 

