Horton

Re: Clean up NamedLWLockTranche stuff

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Sami Imseih <samimseih@gmail.com>, Robert Haas <robertmhaas@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2026-03-27T21:50:12Z
Lists: pgsql-hackers 
On Fri, Mar 27, 2026 at 05:22:33PM -0400, Andres Freund wrote:
> TRAP: failed Assert("MemoryContextIsValid(context)"), File: "mcxt.c", Line: 1270, PID: 230491
> [...](ExceptionalCondition+0x54)[0xaaaae186c204]
> [...](MemoryContextAllocExtended+0x0)[0xaaaae18a2a24]
> [...](RequestNamedLWLockTranche+0x6c)[0xaaaae16e7310]
> [...](process_shmem_requests+0x28)[0xaaaae1881628]
> [...](PostgresSingleUserMain+0xc4)[0xaaaae1701a34]
> [...](main+0x6ac)[0xaaaae12a2adc]
> /lib/aarch64-linux-gnu/libc.so.6(__libc_start_main+0xe8)[0xffff99713dd8]
> [...](+0xf2b98)[0xaaaae12a2b98]
> Aborted
> pg_rewind: error: postgres single-user mode in target cluster failed

Hm.  AFAICT PostmasterContext isn't created in single-user mode, and the
commit in question has RequestNamedLWLockTranche() allocate requests there.
I guess the idea is to allow backends to free that memory after forking
from postmaster, but we don't do that for the NamedLWLockTrancheRequests
list.  Maybe we should surround the last part of that function with
MemoryContextSwitchTo(...) to either TopMemoryContext or PostmasterContext
depending on whether we're in single-user mode.

-- 
nathan

Commits

  1. Fix outdated comment on MainLWLockArray

  2. Fix RequestNamedLWLockTranche in single-user mode

  3. Minor comment fixes to yesterday's LWLock tranche refactoring

  4. Move ShmemIndexLock into ShmemAllocator

  5. Use ShmemInitStruct to allocate lwlock.c's shared memory

  6. Use a separate spinlock to protect LWLockTranches

  7. Refactor how user-defined LWLock tranches are stored in shmem

  8. Rename MAX_NAMED_TRANCHES to MAX_USER_DEFINED_TRANCHES

  9. Fix re-initialization of LWLock-related shared memory.