opal/sync: fix race condition

ompi/request/req_wait.c

@@ -15,6 +15,7 @@
  * Copyright (c) 2012      Oak Ridge National Labs.  All rights reserved.
  * Copyright (c) 2016      Los Alamos National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2016      Mellanox Technologies. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -116,7 +117,8 @@ int ompi_request_default_wait_any(size_t count,
         if (MPI_STATUS_IGNORE != status) {
             *status = ompi_status_empty;
         }
-        WAIT_SYNC_RELEASE(&sync);
+        /* No signal-in-flight can be in this case */
+        WAIT_SYNC_RELEASE_NOWAIT(&sync);
         return rc;
     }
 
@@ -140,6 +142,15 @@ int ompi_request_default_wait_any(size_t count,
             *index = i;
         }
     }
+
+    if( *index == completed ){
+        /* Only one request has triggered. There was no
+         * in-flight completions.
+         * Drop the signalled flag so we won't block
+         * in WAIT_SYNC_RELEASE 
+         */
+        WAIT_SYNC_SIGNALLED(&sync);
+    }        
 
     request = requests[*index];
     assert( REQUEST_COMPLETE(request) );
@@ -361,7 +372,8 @@ int ompi_request_default_wait_some(size_t count,
     ompi_request_t **rptr = NULL;
     ompi_request_t *request = NULL;
     ompi_wait_sync_t sync;
-
+    size_t sync_sets = 0, sync_unsets = 0;
+
     WAIT_SYNC_INIT(&sync, 1);
 
     *outcount = 0;
@@ -386,10 +398,12 @@ int ompi_request_default_wait_some(size_t count,
             num_requests_done++;
         }
     }
+    sync_sets = count - num_requests_null_inactive - num_requests_done;
 
     if(num_requests_null_inactive == count) {
         *outcount = MPI_UNDEFINED;
-        WAIT_SYNC_RELEASE(&sync);
+        /* nobody will signall us */
+        WAIT_SYNC_RELEASE_NOWAIT(&sync);
         return rc;
     }
 
@@ -420,6 +434,14 @@ int ompi_request_default_wait_some(size_t count,
             num_requests_done++;
         }
     }
+    sync_unsets = count - num_requests_null_inactive - num_requests_done;
+
+    if( sync_sets == sync_unsets ){
+        /* nobody knows about us,
+         * set signa-in-progress flag to false
+         */
+        WAIT_SYNC_SIGNALLED(&sync);
+    }
 
     WAIT_SYNC_RELEASE(&sync);
 

opal/threads/wait_sync.h

@@ -5,6 +5,7 @@
  *                         reserved.
  * Copyright (c) 2016      Los Alamos National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2016      Mellanox Technologies. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -24,26 +25,50 @@ typedef struct ompi_wait_sync_t {
     pthread_mutex_t lock;
     struct ompi_wait_sync_t *next;
     struct ompi_wait_sync_t *prev;
+    volatile bool signaling;
 } ompi_wait_sync_t;
 
 #define REQUEST_PENDING        (void*)0L
 #define REQUEST_COMPLETED      (void*)1L
 
 #define SYNC_WAIT(sync)                 (opal_using_threads() ? sync_wait_mt (sync) : sync_wait_st (sync))
 
+/* The loop in release handles a race condition between the signaling
+ * thread and the destruction of the condition variable. The signaling
+ * member will be set to false after the final signaling thread has
+ * finished opertating on the sync object. This is done to avoid
+ * extra atomics in the singalling function and keep it as fast
+ * as possible. Note that the race window is small so spinning here
+ * is more optimal than sleeping since this macro is called in
+ * the critical path. */
 #define WAIT_SYNC_RELEASE(sync)                       \
     if (opal_using_threads()) {                       \
-       pthread_cond_destroy(&(sync)->condition);      \
-       pthread_mutex_destroy(&(sync)->lock);          \
+        while ((sync)->signaling) {                   \
+            continue;                                 \
+        }                                             \
+        pthread_cond_destroy(&(sync)->condition);     \
+        pthread_mutex_destroy(&(sync)->lock);         \
     }
 
+#define WAIT_SYNC_RELEASE_NOWAIT(sync)                \
+    if (opal_using_threads()) {                       \
+        pthread_cond_destroy(&(sync)->condition);     \
+        pthread_mutex_destroy(&(sync)->lock);         \
+    }
+
+
 #define WAIT_SYNC_SIGNAL(sync)                        \
     if (opal_using_threads()) {                       \
         pthread_mutex_lock(&(sync->lock));            \
         pthread_cond_signal(&sync->condition);        \
         pthread_mutex_unlock(&(sync->lock));          \
+        sync->signaling = false;                      \
     }
 
+#define WAIT_SYNC_SIGNALLED(sync){                    \
+        (sync)->signaling = false;                    \
+}        
+
 OPAL_DECLSPEC int sync_wait_mt(ompi_wait_sync_t *sync);
 static inline int sync_wait_st (ompi_wait_sync_t *sync)
 {
@@ -61,6 +86,7 @@ static inline int sync_wait_st (ompi_wait_sync_t *sync)
         (sync)->next = NULL;                                    \
         (sync)->prev = NULL;                                    \
         (sync)->status = 0;                                     \
+        (sync)->signaling = true;                               \
         if (opal_using_threads()) {                             \
             pthread_cond_init (&(sync)->condition, NULL);       \
             pthread_mutex_init (&(sync)->lock, NULL);           \
@@ -81,8 +107,9 @@ static inline void wait_sync_update(ompi_wait_sync_t *sync, int updates, int sta
         }
     } else {
         /* this is an error path so just use the atomic */
-        opal_atomic_swap_32 (&sync->count, 0);
         sync->status = OPAL_ERROR;
+        opal_atomic_wmb ();
+        opal_atomic_swap_32 (&sync->count, 0);
     }
     WAIT_SYNC_SIGNAL(sync);
 }