enc_table which manages Encoding information. rb_encoding_list also manages Encoding objects. Both are accessed/modified by ractors simultaneously so that they should be synchronized. For enc_table, this patch introduced GLOBAL_ENC_TABLE_ENTER/LEAVE/EVAL to access this table with VM lock. To make shortcut, three new global variables global_enc_ascii, global_enc_utf_8, global_enc_us_ascii are also introduced. For rb_encoding_list, we split it to rb_default_encoding_list (256 entries) and rb_additional_encoding_list. rb_default_encoding_list is fixed sized Array so we don't need to synchronized (and most of apps only needs it). To manage 257 or more encoding objects, they are stored into rb_additional_encoding_list. To access rb_additional_encoding_list., VM lock is needed.