ompi/datatype: make datatype pack thread safe

This commit makes ompi_datatype_get_pack_description thread safe. The
call is used by osc/pt2pt to send the packed description to remote
peers. Before this commit if MPI_THREAD_MULTIPLE is enabled and the
user uses MPI_Put, MPI_Get, etc we could hit a race where multiple
threads attempt to store the packed description on the datatype. Since
the code in question is not performance-critical the threading fix
uses opal_atomic_* calls instead of bothering with OPAL_THREAD_*.

Signed-off-by: Nathan Hjelm <[email protected]>

Author: hjelmn

ompi/datatype/ompi_datatype_args.c

@@ -484,17 +484,34 @@ int ompi_datatype_get_pack_description( ompi_datatype_t* datatype,
     void* recursive_buffer;
 
     if( NULL == datatype->packed_description ) {
+        void *packed_description;
+
         if( ompi_datatype_is_predefined(datatype) ) {
-            datatype->packed_description = malloc(2 * sizeof(int));
+            packed_description = malloc(2 * sizeof(int));
         } else if( NULL == args ) {
             return OMPI_ERROR;
         } else {
-            datatype->packed_description = malloc(args->total_pack_size);
+            packed_description = malloc(args->total_pack_size);
         }
-        recursive_buffer = datatype->packed_description;
+        recursive_buffer = packed_description;
         __ompi_datatype_pack_description( datatype, &recursive_buffer, &next_index );
-        if( !ompi_datatype_is_predefined(datatype) ) {
-            args->total_pack_size = (uintptr_t)((char*)recursive_buffer - (char*)datatype->packed_description);
+
+        if (opal_atomic_cmpset (&datatype->packed_description, NULL, packed_description)) {
+            opal_atomic_wmb ();
+            if( !ompi_datatype_is_predefined(datatype) ) {
+                args->total_pack_size = (uintptr_t)((char*)recursive_buffer - (char*)datatype->packed_description);
+            }
+        } else {
+            /* another thread beat us to it */
+            free (packed_description);
+        }
+    }
+
+    if (!ompi_datatype_is_predefined(datatype)) {
+        /* spin until the pack size is updated */
+        while (0 == args->total_pack_size) {
+            struct timespec interval = {.tv_sec = 0, .tv_nsec = 1000};
+            nanosleep (&interval, NULL);
         }
     }