Re: Question on LWLockMode in dsa.c

On Tue, Jan 30, 2024 at 6:24 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> Hi,
>
> While working on radix tree patch[1], John Naylor found that dsa.c
> doesn't already use shared locks even in dsa_dump(). dsa_dump() seems
> a pure read-only function so I thought we could use a shared lock mode
> there. Is there any reason to use exclusive mode even in dsa_dump()?

check_for_freed_segments_locked which is called from dsa_dump does
mark a few variables of the dsa area to NULL right? Are you suggesting
to take initially a shared lock in dsa_dump and upgrade it to
exclusive mode in the unlikely event of
unlikely(area->freed_segment_counter != freed_segment_counter)
occurring in check_for_freed_segments_locked?

> Ultimately, since we're trying to add a new function
> dsa_get_total_size() that just returns
> dsa_area_control.total_segment_size and therefore would also be a
> read-only function, I'd like to find out the correct lock mode there.
>
> [1] https://www.postgresql.org/message-id/CANWCAZYYzoKp_4%2B1m5mn-TRD62BTwom8iLXLOWMsHkkwFi%3Drzg%40mail.gmail.com

Do you see any lock contention or any other issues with exclusive
locks on dsa areas?

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com