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Re: Buffer locking is special (hints, checksums, AIO writes)

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Alexander Lakhin <exclusion@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Melanie Plageman <melanieplageman@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-01-24T21:11:47Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> I think this is more likely to be a spgist bug, not a bug in the patch.  From
> what I can tell, spgist tries to conditionally lock a buffer that it itself
> already has locked exclusively - that's why the assertion is failing.

I dunno.  It looks to me like the previous LWLock-based implementation
of ConditionalLockBuffer() had no such restriction as

	/*
	 * We better not already hold a lock on the buffer.
	 */
	Assert(entry->data.lockmode == BUFFER_LOCK_UNLOCK);

Maybe I'm missing something, but it looks like the old code would
return false if the buffer was already locked, whether that lock
was held by our process or another one.  SPGist evidently has an
assumption in edge cases that that is the behavior, and I'm not
convinced that it's a good idea to change it.  There may be other
such edge cases we've not tripped over yet.

> We could of course just accept this case and have the conditional lock
> acquisition fail, but I think trying to conditionally lock a buffer that you
> already lock is indicative of something having gone wrong.

I don't really buy this argument.  Yes, within a single function it'd
be silly to lock a buffer and immediately try to lock it again, but
when you consider cases like recursive modifications of index state,
it's *far* from obvious that some lower recursion level might not try
to lock a buffer that some outer level already locked.  In the case at
hand I think it is probably driven by two recursion levels trying to
acquire free space out of the same buffer.  SPGist is expecting the
lower level to fail to get the lock and then go find some free space
elsewhere.  Yeah, we could probably re-code it to get that outcome
in another way, but why?

			regards, tom lane

Commits

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

  1. bufmgr: Fix ordering of checks in PinBuffer()

  2. Use UnlockReleaseBuffer() in more places

  3. bufmgr: Make UnlockReleaseBuffer() more efficient

  4. bufmgr: Don't copy pages while writing out

  5. Fix use of wrong variable in _hash_kill_items()

  6. Fix bug due to confusion about what IsMVCCSnapshot means

  7. bufmgr: Switch to standard order in MarkBufferDirtyHint()

  8. bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

  9. Require share-exclusive lock to set hint bits and to flush

  10. heapam: Don't mimic MarkBufferDirtyHint() in inplace updates

  11. bufmgr: Allow conditionally locking of already locked buffer

  12. bufmgr: Avoid spurious compiler warning after fcb9c977aa5

  13. lwlock: Remove ForEachLWLockHeldByMe

  14. lwlock: Remove support for disowned lwlwocks

  15. bufmgr: Implement buffer content locks independently of lwlocks

  16. bufmgr: Change BufferDesc.state to be a 64-bit atomic

  17. lwlock: Improve local variable name

  18. lwlock: Invert meaning of LW_FLAG_RELEASE_OK

  19. bufmgr: Make definitions related to buffer descriptor easier to modify

  20. heapam: Add batch mode mvcc check and use it in page mode

  21. freespace: Don't modify page without any lock

  22. heapam: Move logic to handle HEAP_MOVED into a helper function

  23. bufmgr: Optimize & harmonize LockBufHdr(), LWLockWaitListLock()

  24. bufmgr: Add one-entry cache for private refcount

  25. bufmgr: Separate keys for private refcount infrastructure

  26. Add pg_atomic_unlocked_write_u64

  27. Rename BUFFERPIN wait event class to BUFFER

  28. bufmgr: Turn BUFFER_LOCK_* into an enum

  29. lwlock: Fix, currently harmless, bug in LWLockWakeup()

  30. bufmgr: Use atomic sub for unpinning buffers

  31. bufmgr: Allow some buffer state modifications while holding header lock

  32. bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

  33. bufmgr: Don't lock buffer header in StrategyGetBuffer()

  34. bufmgr: fewer calls to BufferDescriptorGetContentLock

  35. bufmgr: Fix signedness of mask variable in BufferSync()

  36. bufmgr: Introduce FlushUnlockedBuffer

  37. Improve ReadRecentBuffer() scalability