220. Lock

MPI_WIN_LOCK(lock_type, rank, assert, win)
IN lock_type	either MPI_LOCK_EXCLUSIVE or MPI_LOCK_SHARED (state)
IN rank	rank of locked window (nonnegative integer)
IN assert	program assertion (integer)
IN win	window object (handle)

int MPI_Win_lock(int lock_type, int rank, int assert, MPI_Win win)

MPI_WIN_LOCK(LOCK_TYPE, RANK, ASSERT, WIN, IERROR)
INTEGER LOCK_TYPE, RANK, ASSERT, WIN, IERROR

void MPI::Win::Lock(int lock_type, int rank, int assert) const

Starts an RMA access epoch. Only the window at the process with rank rank can be accessed by RMA operations on win during that epoch.

MPI_WIN_UNLOCK(rank, win)
IN rank	rank of window (nonnegative integer)
IN win	window object (handle)

int MPI_Win_unlock(int rank, MPI_Win win)

MPI_WIN_UNLOCK(RANK, WIN, IERROR)
INTEGER RANK, WIN, IERROR

void MPI::Win::Unlock(int rank) const

Completes an RMA access epoch started by a call to MPI_WIN_LOCK(...,win). RMA operations issued during this period will have completed both at the origin and at the target when the call returns.

Locks are used to protect accesses to the locked target window effected by RMA calls issued between the lock and unlock call, and to protect local load/store accesses to a locked local window executed between the lock and unlock call. Accesses that are protected by an exclusive lock will not be concurrent at the window site with other accesses to the same window that are lock protected. Accesses that are protected by a shared lock will not be concurrent at the window site with accesses protected by an exclusive lock to the same window.

It is erroneous to have a window locked and exposed (in an exposure epoch) concurrently. I.e., a process may not call MPI_WIN_LOCK to lock a target window if the target process has called MPI_WIN_POST and has not yet called MPI_WIN_WAIT; it is erroneous to call MPI_WIN_POST while the local window is locked.

Rationale.

An alternative is to require MPI to enforce mutual exclusion between exposure epochs and locking periods. But this would entail additional overheads when locks or active target synchronization do not interact in support of those rare interactions between the two mechanisms. The programming style that we encourage here is that a set of windows is used with only one synchronization mechanism at a time, with shifts from one mechanism to another being rare and involving global synchronization. ( End of rationale.)

Advice to users.

Users need to use explicit synchronization code in order to enforce mutual exclusion between locking periods and exposure epochs on a window. ( End of advice to users.)

Implementors may restrict the use of RMA communication that is synchronized by lock calls to windows in memory allocated by MPI_ALLOC_MEM (Section Memory Allocation , page Memory Allocation ). Locks can be used portably only in such memory.

Rationale.

The implementation of passive target communication when memory is not shared requires an asynchronous agent. Such an agent can be implemented more easily, and can achieve better performance, if restricted to specially allocated memory. It can be avoided altogether if shared memory is used. It seems natural to impose restrictions that allows one to use shared memory for 3-rd party communication in shared memory machines.

The downside of this decision is that passive target communication cannot be used without taking advantage of nonstandard Fortran features: namely, the availability of C-like pointers; these are not supported by some Fortran compilers (g77 and Windows/NT compilers, at the time of writing). Also, passive target communication cannot be portably targeted to COMMON blocks, or other statically declared Fortran arrays. ( End of rationale.)
Consider the sequence of calls in the example below.
Example

MPI_Win_lock(MPI_LOCK_EXCLUSIVE, rank, assert, win) 
MPI_Put(..., rank, ..., win) 
MPI_Win_unlock(rank, win) 

The call to MPI_WIN_UNLOCK will not return until the put transfer has completed at the origin and at the target. This still leaves much freedom to implementors. The call to MPI_WIN_LOCK may block until an exclusive lock on the window is acquired; or, the call MPI_WIN_LOCK may not block, while the call to MPI_PUT blocks until a lock is acquired; or, the first two calls may not block, while MPI_WIN_UNLOCK blocks until a lock is acquired --- the update of the target window is then postponed until the call to MPI_WIN_UNLOCK occurs. However, if the call to MPI_WIN_LOCK is used to lock a local window, then the call must block until the lock is acquired, since the lock may protect local load/store accesses to the window issued after the lock call returns.