Date: Thu, 4 Sep 2025 08:51:21 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Rahila Syed <rahilasyed90@gmail.com>
Cc: Sami Imseih <samimseih@gmail.com>,
	Bertrand Drouvot <bertranddrouvot.pg@gmail.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Improve LWLock tranche name visibility across backends
Message-ID: <aLmZWcqRiPY7QRi-@nathan>
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On Thu, Sep 04, 2025 at 03:29:52PM +0530, Rahila Syed wrote:
> Since updates to LocalLWLockCounter and LWLockTrancheNames are performed
> while holding the lock, but reading LocalLWLockCounter and then
> LWLockTrancheNames in GetLWTrancheName can occur without acquiring the
> same lock in case trancheID < LocalLWLockCounter, There is a small window
> between updating LocalLWLockCounter and adding the name to
> LWLockTrancheNames. During this window, if GetLWTrancheNames is called,
> it might attempt to access a name in LWLockTrancheNames that hasn't been
> added yet.

We hold the lock when reading/writing the shared-memory LWLockCounter.
There's no need to take a lock when reading the backend-local
LocalLWLockCounter, as it can't be modified by any other process.
Additionally, when adding a new tranche name, we hold the lock the entire
time while we bump the shared-memory counter and copy the name, so there's
no chance another backend will see the counter update but not the name or
attempt to use the same LWLockTrancheNames slot.

-- 
nathan