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MDEV-10653: SHOW SLAVE STATUS Can Deadlock an Errored Slave

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AKA rpl.rpl_parallel, binlog_encryption.rpl_parallel fails in
buildbot with timeout in include

A replication parallel worker thread can deadlock with another
connection running SHOW SLAVE STATUS. That is, if the replication
worker thread is in do_gco_wait() and is killed, it will already
hold the LOCK_parallel_entry, and during error reporting, try to
grab the err_lock. SHOW SLAVE STATUS, however, grabs these locks in
reverse order. It will initially grab the err_lock, and then try to
grab LOCK_parallel_entry. This leads to a deadlock when both threads
have grabbed their first lock without the second.

This patch implements the MDEV-31894 proposed fix to optimize the
workers_idle() check to compare the last in-use relay log’s
queued_count==dequeued_count for idleness. This removes the need for
workers_idle() to grab LOCK_parallel_entry, as these values are
atomically updated.

Huge thanks to Kristian Nielsen for diagnosing the problem!

Reviewed By:
============
Kristian Nielsen <knielsen@knielsen-hq.org>
Andrei Elkin <andrei.elkin@mariadb.com>
This commit is contained in:
Brandon Nesterenko
2023-11-29 06:53:31 -07:00
parent 5ca63b2b8b
commit 8dad51481b
6 changed files with 203 additions and 19 deletions
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3
sql/rpl_parallel.h
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@ -369,9 +369,10 @@ struct rpl_parallel {
rpl_parallel_entry *find(uint32 domain_id);
void wait_for_done(THD *thd, Relay_log_info *rli);
void stop_during_until();
bool workers_idle();
int wait_for_workers_idle(THD *thd);
int do_event(rpl_group_info *serial_rgi, Log_event *ev, ulonglong event_size);
static bool workers_idle(Relay_log_info *rli);
};