Concurrent Access to Arrays.

In JNI-based wrapper implementations it may be necessary to impose some non-interference rules for concurrent read and write operations on arrays. When an array is passed to an MPI method such as a send or receive operation, the wrapper code will probably extract a pointer to the contents of the array using a JNI Get...ArrayElements routine. If the garbage collector does not support ``pinning'' (temporarily disabling run-time relocation of data for specific arrays--see [#!GORDON!#] for more discussion), the pointer returned by this Get function may be to a temporary copy of the elements. The copy will be written back to the true Java array when a subsequent call to Release...ArrayElements is made. If two operations involving the same array are active concurrently, this copy-back may result in failure to save modifications made by one or more of the concurrent calls.

Such an implementation may have to enforce a safety rule such as: when several MPI send or receive (etc) operations are active concurrently, if any one of those operations writes to a particular array, none of the other operations must read or write any portion of that array.

If the garbage collector supports pinning, this problem does not arise.