Fix a potential race condition in which a progress matching thread could match a request while we are cancelling it.

ompi/mca/pml/ob1/pml_ob1_recvreq.c

@@ -103,13 +103,14 @@ static int mca_pml_ob1_recv_request_cancel(struct ompi_request_t* ompi_request,
     ompi_communicator_t *comm = request->req_recv.req_base.req_comm;
     mca_pml_ob1_comm_t *ob1_comm = comm->c_pml_comm;
 
+    /* The rest should be protected behind the match logic lock */
+    OPAL_THREAD_LOCK(&ob1_comm->matching_lock);
     if( true == request->req_match_received ) { /* way to late to cancel this one */
+        OPAL_THREAD_UNLOCK(&ob1_comm->matching_lock);
         assert( OMPI_ANY_TAG != ompi_request->req_status.MPI_TAG ); /* not matched isn't it */
         return OMPI_SUCCESS;
     }
 
-    /* The rest should be protected behind the match logic lock */
-    OPAL_THREAD_LOCK(&ob1_comm->matching_lock);
     if( request->req_recv.req_base.req_peer == OMPI_ANY_SOURCE ) {
         opal_list_remove_item( &ob1_comm->wild_receives, (opal_list_item_t*)request );
     } else {