my_malloc_size_cb_func() can be called from contexts where it is not safe to
wait for LOCK_thd_kill, for example while holding LOCK_plugin. This could
lead to (probably very unlikely) deadlock of the server.
Fix by skipping the enforcement of --max-session-mem-used in the rare cases
when LOCK_thd_kill cannot be obtained. The limit will instead be enforced on
the following memory allocation. This does not significantly degrade the
behaviour of --max-session-mem-used; that limit is in any case only enforced
"softly", not taking effect until the next point at which the thread does a
check_killed().
Signed-off-by: Kristian Nielsen
---
sql/mysqld.cc | 43 +++++++++++++++++++++++++++++--------------
1 file changed, 29 insertions(+), 14 deletions(-)
diff --git a/sql/mysqld.cc b/sql/mysqld.cc
index 5713045056d..50b2698ccba 100644
--- a/sql/mysqld.cc
+++ b/sql/mysqld.cc
@@ -3789,20 +3789,35 @@ static void my_malloc_size_cb_func(long long size, my_bool is_thread_specific)
thd->status_var.local_memory_used > (int64)thd->variables.max_mem_used &&
likely(!thd->killed) && !thd->get_stmt_da()->is_set())
{
- /* Ensure we don't get called here again */
- char buf[50], *buf2;
- thd->set_killed(KILL_QUERY);
- my_snprintf(buf, sizeof(buf), "--max-session-mem-used=%llu",
- thd->variables.max_mem_used);
- if ((buf2= (char*) thd->alloc(256)))
- {
- my_snprintf(buf2, 256, ER_THD(thd, ER_OPTION_PREVENTS_STATEMENT), buf);
- thd->set_killed(KILL_QUERY, ER_OPTION_PREVENTS_STATEMENT, buf2);
- }
- else
- {
- thd->set_killed(KILL_QUERY, ER_OPTION_PREVENTS_STATEMENT,
- "--max-session-mem-used");
+ /*
+ Ensure we don't get called here again.
+
+ It is not safe to wait for LOCK_thd_kill here, as we could be called
+ from almost any context. For example while LOCK_plugin is being held;
+ but THD::awake() locks LOCK_thd_kill and LOCK_plugin in the opposite
+ order (MDEV-33443).
+
+ So ignore the max_mem_used limit in the unlikely case we cannot obtain
+ LOCK_thd_kill here (the limit will be enforced on the next allocation).
+ */
+ if (!mysql_mutex_trylock(&thd->LOCK_thd_kill)) {
+ char buf[50], *buf2;
+ thd->set_killed_no_mutex(KILL_QUERY);
+ my_snprintf(buf, sizeof(buf), "--max-session-mem-used=%llu",
+ thd->variables.max_mem_used);
+ if ((buf2= (char*) thd->alloc(256)))
+ {
+ my_snprintf(buf2, 256,
+ ER_THD(thd, ER_OPTION_PREVENTS_STATEMENT), buf);
+ thd->set_killed_no_mutex(KILL_QUERY,
+ ER_OPTION_PREVENTS_STATEMENT, buf2);
+ }
+ else
+ {
+ thd->set_killed_no_mutex(KILL_QUERY, ER_OPTION_PREVENTS_STATEMENT,
+ "--max-session-mem-used");
+ }
+ mysql_mutex_unlock(&thd->LOCK_thd_kill);
}
}
DBUG_ASSERT((longlong) thd->status_var.local_memory_used >= 0 ||
--
2.30.2
