Horton

Re: Improve LWLock tranche name visibility across backends

Sami Imseih <samimseih@gmail.com>

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From: Sami Imseih <samimseih@gmail.com>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Rahila Syed <rahilasyed90@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2025-08-29T13:30:06Z
Lists: pgsql-hackers

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> On Thu, Aug 28, 2025 at 05:53:23PM -0500, Sami Imseih wrote:
> > Just a few things that were discussed earlier, that I incorporated now.
> >
> > 1/ We should be checking that tranche_name is NOT NULL when
> > LWLockNewTrancheId or RequestNamedLWLockTranche is called.
>
> Right, if not strlen() does segfault.
>
> In addition to checking for NULL, should we also check for empty string? Currently,
> the patch does accept strlen(tranche_name) == 0.

I am not inclined to prevent an empty string. It's currently allowed and rather
not change that.

> ```
> typedef struct NamedLWLockTranche
> {
>    char        trancheName[NAMEDATALEN];
>    int            num_lwlocks;
> } NamedLWLockTranche;
> ```
> if there is no interest to backpatch [0], maybe we should just make this
> change as part of this patch set. What do you think? I can make this change
> in v18.

Here is v18. It includes a third patch to fix the issue identified in
[0], which can
be applied to HEAD as part of this thread. If we want to backpatch the stable
branches, the version in [0] is suitable.

Note that I created a LWLockNewTrancheIdInternal which takes a tranch
name and number of lwlocks. The Internal version is used during startup when
requested lwlocks are appended to shared memory, and the existing
LWLockNewTrancheId  calls the internal version with 0 lwlocks.
This keeps all the logic to appending a new tranche ( while holding
the spinlock )
in the same routine.


--
Sami

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().