Hi Sergei,After QA runs done by Ramesh, we now know the latest fix candidate i.e. what is in bb-10.2-MDEV-25114-galera-v2 is incorrect. Problem is in wsrep_close_connections() as it holds LOCK_thread_count while it does abort_replicated that will call wsrep_abort_transaction and there we use find_thread_by_id that would also take LOCK_thread_count. As there is another code path here, the problem is not easily fixed. We can't just release LOCK_thread_count at wsrep_close_connections as we iterate the thread list.I must say I'm not sure what to do now.(gdb) bt
#0 __pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ../sysdeps/unix/sysv/linux/pthread_kill.c:56
#1 0x000056333963a2e8 in my_write_core (sig=sig@entry=6) at /test/mtest/10.2_dbg/mysys/stacktrace.c:382
#2 0x0000563338f2993d in handle_fatal_signal (sig=6) at /test/mtest/10.2_dbg/sql/signal_handler.cc:355
#3 <signal handler called>
#4 __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
#5 0x000014b799572859 in __GI_abort () at abort.c:79
#6 0x000056333963edc4 in safe_mutex_lock (mp=0x563339e47220 <LOCK_thread_count>, my_flags=my_flags@entry=0,
file=file@entry=0x5633396d9050 "/test/mtest/10.2_dbg/sql/sql_parse.cc", line=line@entry=8902)
at /test/mtest/10.2_dbg/mysys/thr_mutex.c:264
#7 0x0000563338d1aea7 in inline_mysql_mutex_lock (src_line=8902, src_file=0x5633396d9050 "/test/mtest/10.2_dbg/sql/sql_parse.cc",
that=<optimized out>) at /test/mtest/10.2_dbg/include/mysql/psi/mysql_thread.h:688
#8 find_thread_by_id (id=id@entry=48, query_id=query_id@entry=false) at /test/mtest/10.2_dbg/sql/sql_parse.cc:8902
#9 0x0000563339134c39 in wsrep_abort_transaction (hton=<optimized out>, bf_thd=0x14b680000d90, victim_thd=<optimized out>,
signal=<optimized out>) at /test/mtest/10.2_dbg/storage/innobase/handler/ha_innodb.cc:19821
#10 0x0000563338f38cbf in ha_abort_transaction (bf_thd=bf_thd@entry=0x14b680000d90, victim_thd=victim_thd@entry=0x14b680000d90,
signal=signal@entry=1 '\001') at /test/mtest/10.2_dbg/sql/handler.cc:6327
#11 0x0000563338ebed6d in wsrep_abort_thd (bf_thd_ptr=bf_thd_ptr@entry=0x14b680000d90, victim_thd_ptr=victim_thd_ptr@entry=0x14b680000d90,
signal=signal@entry=1 '\001') at /test/mtest/10.2_dbg/sql/wsrep_thd.cc:832
#12 0x0000563338eaa2bd in abort_replicated (thd=thd@entry=0x14b680000d90) at /test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:2269
#13 0x0000563338eae097 in wsrep_close_client_connections (wait_to_end=wait_to_end@entry=1 '\001',
except_caller_thd=except_caller_thd@entry=0x0) at /test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:2437
#14 0x0000563338eaedf6 in wsrep_stop_replication (thd=thd@entry=0x0) at /test/mtest/10.2_dbg/sql/wsrep_mysqld.cc:962
#15 0x0000563338c543d8 in kill_server (sig_ptr=sig_ptr@entry=0x0) at /test/mtest/10.2_dbg/sql/mysqld.cc:2009
#16 0x0000563338c558d5 in kill_server_thread (arg=<optimized out>) at /test/mtest/10.2_dbg/sql/mysqld.cc:2047
#17 0x000014b799a7a609 in start_thread (arg=<optimized out>) at pthread_create.c:477
#18 0x000014b79966f293 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95R: JanOn Wed, Oct 6, 2021 at 5:03 PM Sergei Golubchik <serg@mariadb.org> wrote:Hi, Jan!
On Oct 06, Jan Lindström wrote:
> >
> > > > > +/* This is wrapper for wsrep_break_lock in thr_lock.c */
> > > > > +static int wsrep_thr_abort_thd(void *bf_thd_ptr, void *victim_thd_ptr, my_bool signal)
> > > > > +{
> > > > > + THD* victim_thd= (THD *) victim_thd_ptr;
> > > > > + /* We need to lock THD::LOCK_thd_data to protect victim
> > > > > + from concurrent usage or disconnect or delete. */
> > > >
> > > > How do you know victim_thd wasn't deleted before you locked
> > > > LOCK_thd_data below?
> > >
> > > I must say the thr_lock code is not familiar to me but there are
> > > mysql_mutex_lock() calls to lock->mutex. After code review it is not
> > > clear to me what that mutex is.
> >
> > where are mysql_mutex_lock() calls to lock->mutex?
>
> mysys/thr_lock.c there is function thr_lock() there is call to
> mysql_mutex_lock(&lock->mutex); this is before wsrep_break_lock where we
> call wsrep_thd_abort
this is for table locks. `lock` is `data->lock` where `data` is THR_LOCK
structure somewhere in the table share. It locks tables and handlers,
not threads. And InnoDB isn't using it at all anyway.
>
> > > > > if (victim_trx) {
> > > > > + wsrep_thd_UNLOCK(victim_thd);
> > > >
> > > > what keeps victim_trx from disappearing here?
> > >
> > > Nothing. Do you have suggestions ?
> >
> > A couple of thoughts:
> >
> > * Why do you unlock LOCK_thd_data here at all? I believed the whole
> > point of using TOI was to make sure that even if you lock mutexes in
> > a different order, it will not cause a deadlock.
>
> This is DDL-case when we have a MDL-conflict not user KILL, we need to
> release it in my opinion because we need to take mutexes in
> lock_sys -> trx -> THD::LOCK_thd_data order,
> if I do not release I can see easily safe_mutex warnings
I don't understand. First, lock_sys and trx mutexes are not covered by
safe_mutex, so it cannot possibly warn about them.
Second, the reason for taking mutexes always in the same order is to
avoid deadlocks. And yor TOI trick should archieve that.
> > * You cannot pass a THD safely over a gap where no locks keep it in
> > existence. You can pass an integer, thread id, and use
> > find_thread_by_id later.
>
> I have a suggestion for this:
>
> if (victim_trx) {
> +
> + const trx_id_t victim_trx_id= victim_trx->id;
> + const longlong victim_thread= thd_get_thread_id(victim_thd);
> lock_mutex_enter();
> - trx_mutex_enter(victim_trx);
> - wsrep_innobase_kill_one_trx(bf_thd, bf_trx, victim_trx, signal);
> + if (trx_t* victim= trx_rw_is_active(victim_trx_id, NULL, true)) // If this succeeds, trx can not go away
> + {
> + trx_mutex_enter(victim);
> + ut_ad(victim->mysql_thd ?
> + thd_get_thread_id(victim->mysql_thd) == victim_thread :
> + 1);
> + wsrep_innobase_kill_one_trx(bf_thd, bf_trx, victim, signal);
> + trx_mutex_exit(victim);
> + victim->release_reference(); // Now trx can go away after we have released lock_sys
> + wsrep_srv_conc_cancel_wait(victim);
> + }
> lock_mutex_exit();
> - trx_mutex_exit(victim_trx);
> - wsrep_srv_conc_cancel_wait(victim_trx);
> DBUG_VOID_RETURN;
I don't understand, sorry. It's not quite clear what this patch should
apply to. May be you can paste what the new code should look like, not a
patch?
In particular, where are wsrep_thd_UNLOCK and wsrep_thd_LOCK. Are they
present in this new variant at all?
>
> > By the way, how long can WSREP_TO_ISOLATION_BEGIN() take?
> > Does it need to wait for everything else being replicated?
>
> As long as it takes to start it on all nodes in the cluster.
So, KILL basically won't work when a cluster is starting.
Can bf aborts happen during a cluster startup?
Regards,
Sergei
VP of MariaDB Server Engineering
and security@mariadb.org