Re: Connections hang indefinitely while taking a gin index's LWLock buffer_content lock(PG10.7)

On Sun, Sep 29, 2019 at 5:38 PM Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> On Sun, Sep 29, 2019 at 11:17 AM chenhj <chjischj(at)163(dot)com> wrote:
> > Does the locking order of autovacuum process(root->right->left) correct? While insert process lock gin buffer by order of bottom->top and left->right.
> >
> > 1. vacuum(root->right->left):
>
> Starting from root seems OK for me, because vacuum blocks all
> concurrent inserts before doing this. But this needs to be properly
> documented in readme.
>
> Locking from right to left is clearly wrong. It could deadlock with
> concurrent ginStepRight(), which locks from left to right. I expect
> this happened in your case. I'm going to reproduce this and fix.

I just managed to reproduce this using two sessions on master branch.

session 1
session 2

# create table test with (autovacuum_enabled = false) as (select
array[1] ar from generate_series(1,20000) i);
# create index test_ar_idx on test using gin (ar);
# vacuum analyze test;
# delete from test;

# set enable_seqscan = off;
gdb> b ginbtree.c:150
# select * from test where ar @> '{1}'::integer[];
Step in gdb just before ReadBuffer() in ReleaseAndReadBuffer().

gdb> b ginvacuum.c:155
# vacuum test;

gdb > continue
gdb> continue

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company