[Mpi-forum] Threading guarantees of MPI_User_function

[Phil Ruffwind]

Hello all,

I have another question about MPI_User_function.  Does MPI make any
guarantees regarding (a) whether multiple invocations of
MPI_User_function could occur simultaneously within a given process for
a given MPI_Op, and regarding (b) which thread MPI_User_function gets
called on?

The answer to (a) affects whether MPI_User_function would need any
synchronization (like mutexes or semaphores) if it mutates global state.
 The answer to (b) affects whether it is possible to use thread-local
state inside MPI_User_function.

I would appreciate any clarifications.  Thanks.
Phil