charset.c:

  Bug#12109 possible locking bug in init_available_charset
  Recheck charset_initialized inside locked code,
  to garantee two threads are not entering
  consequently.

mysys/charset.c

@@ -383,7 +383,8 @@ static my_bool init_available_charsets(myf myflags)
       while we may changing the cs_info_table
     */
     pthread_mutex_lock(&THR_LOCK_charset);
-
+    if (!charset_initialized)
+    {
       bzero(&all_charsets,sizeof(all_charsets));
       init_compiled_charsets(myflags);
       
@@ -403,6 +404,7 @@ static my_bool init_available_charsets(myf myflags)
       strmov(get_charsets_dir(fname), MY_CHARSET_INDEX);
       error= my_read_charset_file(fname,myflags);
       charset_initialized=1;
+    }
     pthread_mutex_unlock(&THR_LOCK_charset);
   }
   return error;