ompi/request: fix hang in ompi_request_wait_completion

This commit fixes a hang reported by @nysal which happens when a
request is completed after a sync object is created but before the
sync object can be assigned to the request. In this case we need to
set the sync signaling field to false to ensure WAIT_SYNC_RELEASE does
not hang.

Fixes #1828

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

Author: hjelmn

ompi/request/request.h

@@ -377,12 +377,15 @@ static inline int ompi_request_free(ompi_request_t** request)
 
 static inline void ompi_request_wait_completion(ompi_request_t *req)
 {
-    if (opal_using_threads ()) {
+    if (opal_using_threads () && !REQUEST_COMPLETE(req)) {
         ompi_wait_sync_t sync;
         WAIT_SYNC_INIT(&sync, 1);
 
-        if(OPAL_ATOMIC_CMPSET_PTR(&req->req_complete, REQUEST_PENDING, &sync)) {
+        if (OPAL_ATOMIC_CMPSET_PTR(&req->req_complete, REQUEST_PENDING, &sync)) {
             SYNC_WAIT(&sync);
+        } else {
+            /* completed before we had a chance to swap in the sync object */
+            WAIT_SYNC_SIGNALLED(&sync);
         }
 
         assert(REQUEST_COMPLETE(req));