Re: Connections hang indefinitely while taking a gin index's LWLock buffer_content lock

From: Andres Freund <andres(at)anarazel(dot)de>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>, Peter Geoghegan <pg(at)bowt(dot)ie>, chenhj <chjischj(at)163(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Connections hang indefinitely while taking a gin index's LWLock buffer_content lock
Date: 2018-12-13 19:46:11
Message-ID: 20181213194611.zttrlelwi7qu2ewc@alap3.anarazel.de
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Hi,

On 2018-12-13 22:40:59 +0300, Alexander Korotkov wrote:
> It doesn't mater, because we release all locks on every buffer at one
> time. The unlock order can have effect on what waiter will acquire
> the lock next after ginRedoDeletePage(). However, I don't see why one
> unlock order is better than another. Thus, I just used the rule of
> thumb to not change code when it's not necessary for bug fix.

I think it's right to not change unlock order at the same time as a
bugfix here. More generally I think it can often be useful to default
to release locks in the inverse order they've been acquired - if there's
any likelihood that somebody will acquire them in the same order, that
ensures that such a party would only need to wait for a lock once,
instead of being woken up for one lock, and then immediately having to
wait for the next one.

Greetings,

Andres Freund

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