Horton
Menu

Re: Better shared data structure management and resizable shared data structures

Heikki Linnakangas <hlinnaka@iki.fi>

View thread
From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>, chaturvedipalak1911@gmail.com
Date: 2026-04-21T07:40:14Z
Lists: pgsql-hackers 
On 07/04/2026 17:19, Ashutosh Bapat wrote:
> Hi Heikki,
> CallShmemCallbacksAfterStartup() holds ShmemIndexLock while invoking
> init_fn/attach_fn callbacks. That looks wrong. Before this commit,
> init or attach code was not run with the lock held. Any reason the
> lock is held while calling init and attach callbacks. Since these
> function can come from extensions, we don't have control on what goes
> in those functions, and thus looks problematic. Further, it will
> serialize all the attach_fn executions across backends, since each
> will be run under the lock.

This was intentional, I added a note in the docs about it:

       When <function>RegisterShmemCallbacks()</function> is called after
       startup, it will immediately call the appropriate callbacks, 
depending
       on whether the requested memory areas were already initialized by
       another backend. The callbacks will be called while holding an 
internal
       lock, which prevents concurrent two backends from initializing the
       memory area concurrently.

That "internal lock" is ShmemIndexLock. I piggybacked on that since the 
code needs to acquire it anyway for the hash table lookups.

With the old ShmemInitStruct() interface, extensions needed to do the 
locking themselves, usually by holding AddinShmemInitLock.

(Now that I read that again, the grammar on the last sentence sounds 
awkward...)

> In my case, the init_fn was performing ShmemIndex lookup which
> deadlocked. It's questionable whether init function should lookup
> ShmemIndex but, it's not something that needs to be prohibited
> either.
Yeah I'm curious what the use case is. We could easily introduce another 
lock or reuse AddinShmemInitLock for this.

- Heikki

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Tidy up #ifdef USE_INJECTION_POINTS guards

  2. Convert all remaining subsystems to use the new shmem allocation API

  3. Convert buffer manager to use the new shmem allocation functions

  4. Add alignment option to ShmemRequestStruct()

  5. Convert AIO to use the new shmem allocation functions

  6. Convert SLRUs to use the new shmem allocation functions

  7. Refactor shmem initialization code in predicate.c

  8. Use the new shmem allocation functions in a few core subsystems

  9. Convert lwlock.c to use the new shmem allocation functions

  10. Introduce a registry of built-in shmem subsystems

  11. Convert pg_stat_statements to use the new shmem allocation functions

  12. Add a test module to test after-startup shmem allocations

  13. Introduce a new mechanism for registering shared memory areas

  14. Move some code from shmem.c and shmem.h

  15. Improve test_lwlock_tranches

  16. Test pg_stat_statements across crash restart

  17. Refactor PredicateLockShmemInit to not reuse var for different things

  18. Refactor ShmemIndex initialization

  19. Add a new shmem_request_hook hook.