upstream patch, provided by Jiri Polach. Re #733

This commit was SVN r16691.

ompi/mca/io/romio/romio/adio/common/ad_open.c

@@ -172,6 +172,37 @@ MPI_File ADIO_Open(MPI_Comm orig_comm,
      */
     /* pvfs2 handles opens specially, so it is actually more efficent for that
      * file system if we skip this optimization */
+    /* NFS handles opens especially poorly, so we cannot use this optimization
+     * on that FS */
+    if (fd->file_system == ADIO_NFS) {
+        /* no optimizations for NFS: */
+       if ((access_mode & ADIO_CREATE) && (access_mode & ADIO_EXCL)) {
+         /* the open should fail if the file exists. Only *1* process should
+          check this. Otherwise, if all processes try to check and the file
+          does not exist, one process will create the file and others who
+          reach later will return error. */
+           if(rank == fd->hints->ranklist[0]) {
+                fd->access_mode = access_mode;
+                (*(fd->fns->ADIOI_xxx_Open))(fd, error_code);
+                MPI_Bcast(error_code, 1, MPI_INT, \
+                                fd->hints->ranklist[0], fd->comm);
+                /* if no error, close the file and reopen normally below */
+                if (*error_code == MPI_SUCCESS)
+                        (*(fd->fns->ADIOI_xxx_Close))(fd, error_code);
+           }
+           else MPI_Bcast(error_code, 1, MPI_INT,
+                           fd->hints->ranklist[0], fd->comm);
+           if (*error_code != MPI_SUCCESS) {
+                   goto fn_exit;
+           }
+           else {
+               /* turn off EXCL for real open */
+               access_mode = access_mode ^ ADIO_EXCL;
+           }
+        }
+    } else {
+
+           /* the actual optimized create on one, open on all */
     if (access_mode & ADIO_CREATE && fd->file_system != ADIO_PVFS2) {
        if(rank == fd->hints->ranklist[0]) {
 	   /* remove delete_on_close flag if set */
@@ -201,6 +232,7 @@ MPI_File ADIO_Open(MPI_Comm orig_comm,
 		   access_mode ^= ADIO_EXCL;
        }
     }
+    }
 
     /* if we are doing deferred open, non-aggregators should return now */
     if (fd->hints->deferred_open ) {