ha_innobase::external_lock(...)
{...
 if (lock_type != F_UNLCK) {
 /* MySQL is setting a new table lock */
...
 if (trx->isolation_level == TRX_ISO_SERIALIZABLE //序列化隔離級別
  && m_prebuilt->select_lock_type == LOCK_NONE
  && thd_test_options(thd, OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)) { //且在一個顯式事務(wù)塊內(nèi)部
 
  /* To get serializable execution, we let InnoDB conceptually add 'LOCK IN SHARE MODE' to all SELECTs
  which otherwise would have been consistent reads. An exception is consistent reads in the AUTOCOMMIT=1 mode:
  we know that they are read-only transactions, and they can be serialized also if performed as consistent reads. */
  m_prebuilt->select_lock_type = LOCK_S; //加讀鎖,即 'LOCK IN SHARE MODE'
  m_prebuilt->stored_select_lock_type = LOCK_S;
 } //否則,不加鎖(這一點(diǎn)也很重要)
...
 } else {
 TrxInInnoDB::end_stmt(trx);
 DEBUG_SYNC_C("ha_innobase_end_statement");
 }
...}

ha_innobase::store_lock(
...
 /* Check for FLUSH TABLES ... WITH READ LOCK */
 if (trx->isolation_level == TRX_ISO_SERIALIZABLE) {
  m_prebuilt->select_lock_type = LOCK_S;
  m_prebuilt->stored_select_lock_type = LOCK_S;
 } else {
  m_prebuilt->select_lock_type = LOCK_NONE;
  m_prebuilt->stored_select_lock_type = LOCK_NONE;
 } 
...
}