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Revert THD::THD(skip_global_sys_var_lock) argument

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Originally introduced by e972125f1 to avoid harmless wait for
LOCK_global_system_variables in a newly created thread, which creation was
initiated by system variable update.

At the same time it opens dangerous hole, when system variable update
thread already released LOCK_global_system_variables and ack_receiver
thread haven't yet completed new THD construction. In this case THD
constructor goes completely unprotected.

Since ack_receiver.stop() waits for the thread to go down, we have to
temporarily release LOCK_global_system_variables so that it doesn't
deadlock with ack_receiver.run(). Unfortunately it breaks atomicity
of rpl_semi_sync_master_enabled updates and makes them not serialized.

LOCK_rpl_semi_sync_master_enabled was introduced to workaround the above.
TODO: move ack_receiver start/stop into repl_semisync_master
enable_master/disable_master under LOCK_binlog protection?

Part of MDEV-14984 - regression in connect performance
This commit is contained in:
Sergey Vojtovich
2019-03-20 18:35:20 +04:00
parent 894df7edb6
commit 779fb636da
8 changed files with 29 additions and 29 deletions
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12
sql/sql_class.cc
View File

@ -597,7 +597,7 @@ extern "C" void thd_kill_timeout(THD* thd)
thd->awake(KILL_TIMEOUT);
}
THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
THD::THD(my_thread_id id, bool is_wsrep_applier)
:Statement(&main_lex, &main_mem_root, STMT_CONVENTIONAL_EXECUTION,
/* statement id */ 0),
rli_fake(0), rgi_fake(0), rgi_slave(NULL),
@ -792,7 +792,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier, bool skip_global_sys_var_lock)
/* Call to init() below requires fully initialized Open_tables_state. */
reset_open_tables_state(this);
init(skip_global_sys_var_lock);
init();
#if defined(ENABLED_PROFILING)
profiling.set_thd(this);
#endif
@ -1167,11 +1167,10 @@ const Type_handler *THD::type_handler_for_date() const
Init common variables that has to be reset on start and on change_user
*/
void THD::init(bool skip_lock)
void THD::init()
{
DBUG_ENTER("thd::init");
if (!skip_lock)
mysql_mutex_lock(&LOCK_global_system_variables);
mysql_mutex_lock(&LOCK_global_system_variables);
plugin_thdvar_init(this);
/*
plugin_thd_var_init() sets variables= global_system_variables, which
@ -1184,8 +1183,7 @@ void THD::init(bool skip_lock)
::strmake(default_master_connection_buff,
global_system_variables.default_master_connection.str,
variables.default_master_connection.length);
if (!skip_lock)
mysql_mutex_unlock(&LOCK_global_system_variables);
mysql_mutex_unlock(&LOCK_global_system_variables);
user_time.val= start_time= start_time_sec_part= 0;