[Mpi-forum] MPI_Win_lock_all() ordering question

[Michael Raymond]

   I've got a question about the use of MPI_Win_lock_all() in the face 
of competing MPI_Win_lock(EXCLUSIVE) calls. Consider the following calls 
sequence:


0							1
MPI_Win_lock(EXCLUSIVE, 0)

MPI_Win_lock(EXCLUSIVE, 1)		MPI_Win_lock_all()

....							....
MPI_Win_unlock(1)				MPI_Win_unlock_all()
MPI_Win_unlock(0)

   In this situation 0 has an exclusive lock on itself. Simultaneously, 
0 tries to get 1 exclusively and 1 tries to get a shared lock on 
everyone. If 0 gets lucky, it will get to go first and everything will 
go fine. If OTOH 1 locks itself shared and then tries to lock 0 shared, 
deadlock ensues. You could argue that there should be some global queue 
/ governor that lets 0 go first, but then I could see running into 
scalability problems.

   I can't find any place in the standard that says if the user 
shouldn't do this, or if deadlock is allowed, or if the MPI 
implementation should figure things out on its own. Thoughts?

-- 
Michael A. Raymond
SGI MPT Team Leader
(651) 683-3434