Horton

Re: Improve LWLock tranche name visibility across backends

Nathan Bossart <nathandbossart@gmail.com>

View thread View on postgr.es
From: Nathan Bossart <nathandbossart@gmail.com>
To: Sami Imseih <samimseih@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-26T00:14:40Z
Lists: pgsql-hackers 
On Mon, Aug 25, 2025 at 04:59:41PM -0500, Sami Imseih wrote:
> hmm, can we really avoid a shared lock when reading from shared memory?
> considering access for both reads and writes can be concurrent to shared
> memory. We are also taking an exclusive lock when writing a new tranche.

We probably want to hold a lock while we 1) increment LWLockCounter and
copy a new tranche name to memory and 2) while we copy the current value of
LWLockCounter to our backend-local variable.  Otherwise, AFAICT we don't
need one.  We could probably use ShmemLock for this.

-- 
nathan

Commits

  1. test_dsa: Avoid leaking LWLock tranches.

  2. Teach DSM registry to ERROR if attaching to an uninitialized entry.

  3. Add a test harness for the LWLock tranche code.

  4. Revert recent change to RequestNamedLWLockTranche().

  5. Move dynamically-allocated LWLock tranche names to shared memory.

  6. Prepare DSM registry for upcoming changes to LWLock tranche names.

  7. Add GetNamedDSA() and GetNamedDSHash().