Hi, Sergey! ok to push On Mar 25, Sergey Vojtovich wrote:

revision-id: 0fa832dc426 (mariadb-10.3.12-97-g0fa832dc426) parent(s): 6f14489ad31 author: Sergey Vojtovich <svoj@mariadb.org> committer: Sergey Vojtovich <svoj@mariadb.org> timestamp: 2019-03-21 17:17:26 +0400 message:

Revert THD::THD(skip_global_sys_var_lock) argument

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

Regards, Sergei Chief Architect MariaDB and security@mariadb.org