switch (lock_mode) {
    case AUTOINC_NO_LOCKING://innodb_autoinc_lock_mode=2
        /* Acquire only the AUTOINC mutex. */
        dict_table_autoinc_lock(m_prebuilt->table);
        break;

    case AUTOINC_NEW_STYLE_LOCKING: // innodb_autoinc_lock_mode=1 注意这里没有break 巧妙的完成了逻辑
        /* For simple (single/multi) row INSERTs, we fallback to the
        old style only if another transaction has already acquired
        the AUTOINC lock on behalf of a LOAD FILE or INSERT ... SELECT
        etc. type of statement. */
        if (thd_sql_command(m_user_thd) == SQLCOM_INSERT
            || thd_sql_command(m_user_thd) == SQLCOM_REPLACE) {

            dict_table_t*   ib_table = m_prebuilt->table;

            /* Acquire the AUTOINC mutex. */
            dict_table_autoinc_lock(ib_table);

            /* We need to check that another transaction isn't
            already holding the AUTOINC lock on the table. */
            if (ib_table->n_waiting_or_granted_auto_inc_locks) {
                /* Release the mutex to avoid deadlocks. */
                dict_table_autoinc_unlock(ib_table);
            } else {
                break;
            }
        }
        /* Fall through to old style locking. */

    case AUTOINC_OLD_STYLE_LOCKING://innodb_autoinc_lock_mode=0 触发
        DBUG_EXECUTE_IF("die_if_autoinc_old_lock_style_used",
                ut_ad(0););
        error = row_lock_table_autoinc_for_mysql(m_prebuilt); //这个函数上表上的自增锁

        if (error == DB_SUCCESS) {

            /* Acquire the AUTOINC mutex. */
            dict_table_autoinc_lock(m_prebuilt->table);
        }
        break;

    default:
        ut_error;
    }