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Date: Tue, 09 Jan 2001 13:35:50 -0600
To: mpi-core@XXXXXXXXXXX
From: William Gropp <gropp@XXXXXXXXXXX>
Subject: Are attribute copy functions synchronized?
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Consider the following case:  two threads belonging to the same process both execute MPI_COMM_DUP on different communicators at the same time.  In addition, both of these communicators have an attribute for a particular keyval whose copy function looks something like this:

int attr_copy( MPI_Comm oldcomm, int keyval, void *extra_state, void *attribute_val_in, void *attribute_val_out, int *flag )
{
    int *tag_p = (int *)extra_state;
    int *aval_p;

    aval_p = (int *)malloc( sizeof(int) ); if (!aval_p) MPI_Abort(MPI_COMM_WORLD,0);
    *aval_p = *tag_p;
    *tag_p  = *tag_p + 1;
    *(void **)attribute_val_out = aval_p;
    *flag = 1;
    return MPI_SUCCESS;
}

This attribute is intended to provide an integer sequence number to each communicator, based on a single, global value maintained in the keyval's extra_state.  In the multithreaded case, it is possible for both threads to read the same value from *tag_p.

Is this a user error?  Should a multithreaded MPI ensure that only one thread at a time invokes an attribute copy (or delete) function?

From a user's perspective, I'd answer yes to both - it is a user error, but I'd like the MPI implementation to handle this for me.  A related question is: Is it erroneous for an MPI implementation to ensure that only one thread at a time invokes an attribute copy or delete function?

Bill