Merge istruewing@bk-internal.mysql.com:/home/bk/mysql-5.1-engines

into  blade08.mysql.com:/data0/istruewing/autopush/mysql-5.1-bug25460

storage/myisam/mi_extra.c

@@ -350,11 +350,13 @@ int mi_extra(MI_INFO *info, enum ha_extra_function function, void *extra_arg)
 #ifdef HAVE_MMAP
     pthread_mutex_lock(&share->intern_lock);
     /*
-      Memory map the data file if it is not already mapped and if there
-      are no other threads using this table. intern_lock prevents other
-      threads from starting to use the table while we are mapping it.
+      Memory map the data file if it is not already mapped. It is safe
+      to memory map a file while other threads are using file I/O on it.
+      Assigning a new address to a function pointer is an atomic
+      operation. intern_lock prevents that two or more mappings are done
+      at the same time.
     */
-    if (!share->file_map && (share->tot_locks == 1))
+    if (!share->file_map)
     {
       if (mi_dynmap_file(info, share->state.state.data_file_length))
       {