Date: Sun, 16 Jun 2019 15:04:41 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Oleksii Kliukin <alexk@hintbits.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@lists.postgresql.org
Subject: Re: pgsql: Avoid spurious deadlocks when upgrading a tuple lock
Message-ID: <20190616190441.GA30993@alvherre.pgsql>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20190615224713.GA9414@alvherre.pgsql>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

On 2019-Jun-15, Alvaro Herrera wrote:

> But that's not the danger ... with the current coding, it's initialized
> to false every time through that block; that means the tuple lock will
> never be skipped if we jump back to l3.  So the danger is that the first
> iteration sets the variable, then jumps back; second iteration
> initializes the variable again, so instead of skipping the lock, it
> takes it, causing a spurious deadlock.

So, I'm too lazy today to generate a case that fully reproduces the
deadlock, because you need to stall 's2' a little bit using the
well-known advisory lock trick, but this one hits the code that would
re-initialize the variable.

I'm going to push the change of lifetime of the variable for now.

setup
{
    drop table if exists tlu_job;
    create table tlu_job (id integer primary key, name text);

    insert into tlu_job values(1, 'a');
}


teardown
{
    drop table tlu_job;
}

session "s0"
setup { begin; set deadlock_timeout=1}
step "s0_fornokeyupdate" { select id from tlu_job where id = 1 for no key update; }
step "s0_update" { update tlu_job set name = 's0' where id = 1; }
step "s0_commit" { commit; }

session "s1"
setup { begin; set deadlock_timeout=1}
step "s1_for_key_share" { select id from tlu_job where id = 1 for key share; }
step "s1_for_update" { select id from tlu_job where id = 1 for update; }
step "s1_rollback" { rollback; }

session "s2"
setup { begin; set deadlock_timeout=1}
step "s2_for_key_share" { select id from tlu_job where id = 1 for key share; }
step "s2_for_share" { select id from tlu_job where id = 1 for share; }
step "s2_rollback" { rollback; }

session "s3"
setup { begin; set deadlock_timeout=1}
step "s3_update" { update tlu_job set name = 'c' where id = 1; }
step "s3_rollback" { rollback; }

permutation "s1_for_key_share" "s2_for_key_share" "s0_fornokeyupdate" "s2_for_share" "s0_update" "s0_commit" "s1_rollback" "s2_rollback" "s3_rollback"

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services