ompi_mpi_init: fix race condition

There was a race condition in 35438ae: if multiple threads invoked
ompi_mpi_init() simultaneously (which could happen from both MPI and
OSHMEM), the code did not catch this condition -- Bad Things would
happen.

Now use an atomic cmp/set to ensure that only one thread is able to
advance ompi_mpi_init from NOT_INITIALIZED to INIT_STARTED.

Additionally, change the prototype of ompi_mpi_init() so that
oshmem_init() can safely invoke ompi_mpi_init() multiple times (as
long as MPI_FINALIZE has not started) without displaying an error.  If
multiple threads invoke oshmem_init() simultaneously, one of them will
actually do the initialization, and the rest will loop waiting for it
to complete.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

ompi/mpi/c/init.c

@@ -9,7 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
- * Copyright (c) 2007-2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2007-2018 Cisco Systems, Inc.  All rights reserved
  * Copyright (c) 2007-2008 Sun Microsystems, Inc.  All rights reserved.
  * Copyright (c) 2015      Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
@@ -63,9 +63,9 @@ int MPI_Init(int *argc, char ***argv)
        don't lose anything) */
 
     if (NULL != argc && NULL != argv) {
-        err = ompi_mpi_init(*argc, *argv, required, &provided);
+        err = ompi_mpi_init(*argc, *argv, required, &provided, false);
     } else {
-        err = ompi_mpi_init(0, NULL, required, &provided);
+        err = ompi_mpi_init(0, NULL, required, &provided, false);
     }
 
     /* Since we don't have a communicator to invoke an errorhandler on

ompi/mpi/c/init_thread.c

@@ -13,7 +13,7 @@
  * Copyright (c) 2010      Oak Ridge National Labs.  All rights reserved.
  * Copyright (c) 2015      Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
- * Copyright (c) 2015 Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2015-2018 Cisco Systems, Inc.  All rights reserved
  * Copyright (c) 2016      Los Alamos National Security, LLC. All rights
  *                         reserved.
  * $COPYRIGHT$
@@ -63,9 +63,9 @@ int MPI_Init_thread(int *argc, char ***argv, int required,
        don't lose anything) */
 
     if (NULL != argc && NULL != argv) {
-        err = ompi_mpi_init(*argc, *argv, required, provided);
+        err = ompi_mpi_init(*argc, *argv, required, provided, false);
     } else {
-        err = ompi_mpi_init(0, NULL, required, provided);
+        err = ompi_mpi_init(0, NULL, required, provided, false);
     }
 
     /* Since we don't have a communicator to invoke an errorhandler on

ompi/runtime/mpiruntime.h

@@ -175,6 +175,8 @@ void ompi_mpi_thread_level(int requested, int *provided);
  * @param argv argv, typically from main() (IN)
  * @param requested Thread support that is requested (IN)
  * @param provided Thread support that is provided (OUT)
+ * @param reinit_ok Return successfully (with no error) if someone has
+ * already called ompi_mpi_init().
  *
  * @returns MPI_SUCCESS if successful
  * @returns Error code if unsuccessful
@@ -186,7 +188,8 @@ void ompi_mpi_thread_level(int requested, int *provided);
  *
  * It is permissable to pass in (0, NULL) for (argc, argv).
  */
-int ompi_mpi_init(int argc, char **argv, int requested, int *provided);
+int ompi_mpi_init(int argc, char **argv, int requested, int *provided,
+                  bool reinit_ok);
 
 /**
  * Finalize the Open MPI MPI environment

ompi/runtime/ompi_mpi_init.c

@@ -368,7 +368,8 @@ static void fence_release(int status, void *cbdata)
     OPAL_POST_OBJECT(active);
 }
 
-int ompi_mpi_init(int argc, char **argv, int requested, int *provided)
+int ompi_mpi_init(int argc, char **argv, int requested, int *provided,
+                  bool reinit_ok)
 {
     int ret;
     ompi_proc_t** procs;
@@ -384,29 +385,33 @@ int ompi_mpi_init(int argc, char **argv, int requested, int *provided)
 
     ompi_hook_base_mpi_init_top(argc, argv, requested, provided);
 
-    /* Ensure that we were not already initialized or finalized.
-
-       This lock is held for the duration of ompi_mpi_init() and
-       ompi_mpi_finalize().  Hence, if we get it, then no other thread
-       is inside the critical section (and we don't have to check the
-       *_started bool variables). */
-    opal_atomic_rmb();
-    int32_t state = ompi_mpi_state;
+    /* Ensure that we were not already initialized or finalized. */
+    int32_t expected = OMPI_MPI_STATE_NOT_INITIALIZED;
+    int32_t desired  = OMPI_MPI_STATE_INIT_STARTED;
+    opal_atomic_wmb();
+    if (!opal_atomic_compare_exchange_strong_32(&ompi_mpi_state, &expected,
+                                                desired)) {
+        // If we failed to atomically transition ompi_mpi_state from
+        // NOT_INITIALIZED to INIT_STARTED, then someone else already
+        // did that, and we should return.
+        if (expected >= OMPI_MPI_STATE_FINALIZE_STARTED) {
+            opal_show_help("help-mpi-runtime.txt",
+                           "mpi_init: already finalized", true);
+            return MPI_ERR_OTHER;
+        } else if (expected >= OMPI_MPI_STATE_INIT_STARTED) {
+            // In some cases (e.g., oshmem_shmem_init()), we may call
+            // ompi_mpi_init() multiple times.  In such cases, just
+            // silently return successfully.
+            if (reinit_ok) {
+                return MPI_SUCCESS;
+            }
 
-    if (state >= OMPI_MPI_STATE_FINALIZE_STARTED) {
-        opal_show_help("help-mpi-runtime.txt",
-                       "mpi_init: already finalized", true);
-        return MPI_ERR_OTHER;
-    } else if (state >= OMPI_MPI_STATE_INIT_STARTED) {
-        opal_show_help("help-mpi-runtime.txt",
-                       "mpi_init: invoked multiple times", true);
-        return MPI_ERR_OTHER;
+            opal_show_help("help-mpi-runtime.txt",
+                           "mpi_init: invoked multiple times", true);
+            return MPI_ERR_OTHER;
+        }
     }
 
-    /* Indicate that we have *started* MPI_INIT* */
-    opal_atomic_wmb();
-    opal_atomic_swap_32(&ompi_mpi_state, OMPI_MPI_STATE_INIT_STARTED);
-
     /* Figure out the final MPI thread levels.  If we were not
        compiled for support for MPI threads, then don't allow
        MPI_THREAD_MULTIPLE.  Set this stuff up here early in the

oshmem/runtime/oshmem_shmem_init.c

@@ -147,8 +147,13 @@ int oshmem_shmem_init(int argc, char **argv, int requested, int *provided)
     OMPI_TIMING_INIT(32);
 
     if (!oshmem_shmem_initialized) {
-        if (ompi_mpi_state < OMPI_MPI_STATE_INIT_COMPLETED) {
-            ret = ompi_mpi_init(argc, argv, requested, provided);
+        ret = ompi_mpi_init(argc, argv, requested, provided, true);
+
+        // It's posible that another thread is initializing MPI and
+        // has not completed yet.  Keep checking until it is
+        // completed.
+        while (ompi_mpi_state < OMPI_MPI_STATE_INIT_COMPLETED) {
+            usleep(1);
         }
         OMPI_TIMING_NEXT("ompi_mpi_init");