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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, 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: 2026-01-24T20:31:14Z
Lists: pgsql-hackers 
Hi,

On 2026-01-24 21:00:00 +0200, Alexander Lakhin wrote:
> 16.01.2026 01:02, Tom Lane wrote:
> > Various buildfarm animals are complaining about fcb9c977a,
> > similarly to this from calliphoridae [1]:
>
> I've discovered another anomaly introduced with fcb9c977a, this time
> run-time:

With the other anomaly, you mean the spurious compiler warning or something
else?


> for i in `seq 300`; do
> echo "iteration $i"
>
> echo "
> create table t(f1 text);
> create index on t using spgist(f1);
> insert into t select 'a' from generate_series(1, 9000) g(i);
> vacuum analyze t;
> insert into t select 'b' from generate_series(1, 1000) g(i);
> drop table t;
> " | psql >/dev/null -v ON_ERROR_STOP=1 || break;
> done
>
> fails for me as below:
> ...
> iteration 39
> server closed the connection unexpectedly


Thanks for this report, particularly with the easy reproducer!  How did you
find this?


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 reproduced this locally, and could see in a bt full stack that the buffer
that spgist is trying to lock conditionally, is also referenced as
newInnerBuffer in doPickSplit(). So it's not an issue of bufmgr.c loosing
track of which buffers are locked with what mode.

I haven't yet figured out why spgist ends up with a buffer it already is
using.

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.  But I'm open to
going there anyway, just to avoid causing problems with previously "working"
code.


Greetings,

Andres Freund

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