ha_innobase::external_lock(...)

{...

 if (lock_type != F_UNLCK) {

 /* MySQL is setting a new table lock */

...

 if (trx->isolation_level == TRX_ISO_SERIALIZABLE //序列化隔离级别

  && m_prebuilt->select_lock_type == LOCK_NONE

  && thd_test_options(thd, OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)) { //且在一个显式事务块内部

 

  /* 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; //加读锁，即 'LOCK IN SHARE MODE'

  m_prebuilt->stored_select_lock_type = LOCK_S;

 } //否则，不加锁（这一点也很重要）

...

 } 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;

 } 

...

}