Horton

Re: Improve LWLock tranche name visibility across backends

Nathan Bossart <nathandbossart@gmail.com>

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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: 2025-08-26T20:14:39Z
Lists: pgsql-hackers 
On Tue, Aug 26, 2025 at 02:56:22PM -0500, Sami Imseih wrote:
> Here is v12 that replaces the LWLock to access the shared memory with a
> ShmemLock and implements a local counter.

This looks much closer to what I was imagining.

 		/* Initialize the LWLock tranche for the DSA. */
-		dsa_state->tranche = LWLockNewTrancheId();
+		dsa_state->tranche = LWLockNewTrancheId(dsa_state->tranche_name);
 		sprintf(dsa_state->tranche_name, "%s%s", name, DSMR_DSA_TRANCHE_SUFFIX);
-		LWLockRegisterTranche(dsa_state->tranche, dsa_state->tranche_name);
 
 		/* Initialize the LWLock tranche for the dshash table. */
-		dsh_state->tranche = LWLockNewTrancheId();
+		dsh_state->tranche = LWLockNewTrancheId(dsh_state->tranche_name);
 		strcpy(dsh_state->tranche_name, name);
-		LWLockRegisterTranche(dsh_state->tranche, dsh_state->tranche_name);

We probably need to do the sprintf/strcpy before LWLockNewTrancheId().
Also, I'm thinking we should just use the same tranche for both the DSA and
the dshash table [0] to evade the DSMR_DSA_TRANCHE_SUFFIX problem, i.e.,
those tranche names potentially require more space.

+	/* Space for name of each tranche. */
+	size = add_size(size, mul_size(MAX_NAMED_TRANCHES, MAX_NAMED_TRANCHES));

Presumably one of the mul_size() arguments should be
MAX_NAMED_TRANCHES_NAME_LEN.

+NamedLWLockTrancheArray	"Waiting to access the named LWLock tranches array."

+PG_LWLOCK(54, NamedLWLockTrancheArray)

These can be removed now, right?

[0] https://postgr.es/m/aKzIg1JryN1qhNuy%40nathan

-- 
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().