[MariaDB commits] [PATCH] MDEV-29744: Fix incorrect locking order of LOCK_log/LOCK_commit_ordered and LOCK_global_system_variables

[Kristian Nielsen via commits]

The LOCK_global_system_variables must not be held when taking mutexes
such as LOCK_commit_ordered and LOCK_log, as this causes inconsistent
mutex locking order that can theoretically cause the server to
deadlock.

To avoid this, temporarily release LOCK_global_system_variables in two
system variable update functions, like it is done in many other
places.

Enforce the correct locking order at server startup, to more easily
catch (in debug builds) any remaining wrong orders that may be hidden
elsewhere in the code.

Note that when this is merged to 11.4, similar unlock/lock of
LOCK_global_system_variables must be added in update_binlog_space_limit()
as is done in binlog_checksum_update() and fix_max_binlog_size(), as this
is a new function added in 11.4 that also needs the same fix. Tests will
fail with wrong mutex order until this is done.

Signed-off-by: Kristian Nielsen <kniel...@knielsen-hq.org>

fixup_locking_order

Signed-off-by: Kristian Nielsen <kniel...@knielsen-hq.org>
---
 sql/log.cc      | 5 +++++
 sql/sys_vars.cc | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/sql/log.cc b/sql/log.cc
index 512be2e2a6d..ecb177a34fb 100644
--- a/sql/log.cc
+++ b/sql/log.cc
@@ -3595,6 +3595,9 @@ void MYSQL_BIN_LOG::init_pthread_objects()
 
   mysql_mutex_init(m_key_LOCK_binlog_end_pos, &LOCK_binlog_end_pos,
                    MY_MUTEX_INIT_SLOW);
+
+  /* Fix correct mutex order to catch violations quicker (MDEV-35197). */
+  mysql_mutex_record_order(&LOCK_log, &LOCK_global_system_variables);
 }
 
 
@@ -11753,6 +11756,7 @@ binlog_checksum_update(MYSQL_THD thd, struct 
st_mysql_sys_var *var,
   bool check_purge= false;
   ulong UNINIT_VAR(prev_binlog_id);
 
+  mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_mutex_lock(mysql_bin_log.get_log_lock());
   if(mysql_bin_log.is_open())
   {
@@ -11771,6 +11775,7 @@ binlog_checksum_update(MYSQL_THD thd, struct 
st_mysql_sys_var *var,
   mysql_mutex_unlock(mysql_bin_log.get_log_lock());
   if (check_purge)
     mysql_bin_log.checkpoint_and_purge(prev_binlog_id);
+  mysql_mutex_lock(&LOCK_global_system_variables);
 }
 
 
diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index cb35386f883..115bbdf499b 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -1660,7 +1660,9 @@ Sys_max_binlog_stmt_cache_size(
 
 static bool fix_max_binlog_size(sys_var *self, THD *thd, enum_var_type type)
 {
+  mysql_mutex_unlock(&LOCK_global_system_variables);
   mysql_bin_log.set_max_size(max_binlog_size);
+  mysql_mutex_lock(&LOCK_global_system_variables);
   return false;
 }
 static Sys_var_on_access_global<Sys_var_ulong,
-- 
2.39.5

_______________________________________________
commits mailing list -- commits@lists.mariadb.org
To unsubscribe send an email to commits-le...@lists.mariadb.org