Hi Sergei,

This does not seem to work. Consider following:

CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
INSERT INTO t1 VALUES (1);
connection node_2;
SET AUTOCOMMIT=OFF;
START TRANSACTION;
INSERT INTO t1 VALUES (2);
connection node_2a;
ALTER TABLE t1 ADD COLUMN f2 INTEGER, LOCK=EXCLUSIVE;

Problem seems to be the fact that the MDL-lock acquired by thread executing INSERT that thread executing ALTER wants to be released by killing holder is

not released. We do kill query inside InnoDB. This MDL-code is not familiar to me and I do not yet understand why MDL-lock is not released

R: Jan

On Thu, Oct 14, 2021 at 9:49 PM Sergei Golubchik <serg@mariadb.org> wrote:

Hi, Jan!

Here's an idea of the fix:

Let's always use the KILL mutex locking order, that is

victim_thread->LOCK_thd_data -> lock_sys->mutex -> victim_trx->mutex

For this we need to fix wsrep_abort_transaction(), which is called from the
server, and wsrep_innobase_kill_one_trx(), which is called from BF
thread.

wsrep_abort_transaction() can be fixed by not invoking
wsrep_innobase_kill_one_trx() and always using KILL code path (that is
wsrep_thd_awake) and forcing rollback after the kill.

wsrep_innobase_kill_one_trx() can be fixed by not locking LOCK_thd_data
at all, just don't lock it. We know that the victim waits on a lock
inside InnoDB and we've locked trx mutex and lock_sys mutex. The victim
cannot go away, cannot modify its data, it cannot do anything. So,
LOCK_thd_data doesn't seem to be necessary at that point.

I've attached a demo patch. It compiles, but I didn't try to run it,
it's only to show the idea, not a working fix (I already suspect I
removed too much from wsrep_abort_transaction()). Note it's the patch
for 10.2 at the commit 29bbcac0ee8^ - that is one commit before my fix.

On Oct 12, Jan Lindström wrote:
> Hi Sergei,
>
> Update on wsrep_close_connections problem. My suggestion to fix this issue
> is on
> https://github.com/MariaDB/server/commit/99cbe03a44cc95e6f548550df51e7201ebea3b9d
>
> If you have a better solution, please advise.
>
> R: Jan

Regards,
Sergei
VP of MariaDB Server Engineering
and security@mariadb.org