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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>, Alexander Kuzmenkov <akuzmenkov@tigerdata.com>
Cc: Noah Misch <noah@leadboat.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Melanie Plageman <melanieplageman@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-03-17T20:50:09Z
Lists: pgsql-hackers 
Hi,

On 2026-03-13 11:55:53 -0400, Andres Freund wrote:
> On 2026-03-13 10:00:00 +0200, Alexander Lakhin wrote:
> > Hello Andres,
> > 
> > 12.03.2026 00:40, Andres Freund wrote:
> > > I pushed this and many of the later patches in the series.  Here are updated
> > > versions of the remaining changes.  The last two previously were one commit
> > > with "WIP" in the title. The first one has, I think, not had a lot of review -
> > > but it's also not a complicated change.
> > 
> > I've discovered that starting from 82467f627, the following query:
> > SET cpu_operator_cost = 1000;
> > CREATE TABLE t (i INT);
> > INSERT INTO T SELECT 1 FROM generate_series(1, 1000) a;
> > CREATE INDEX hi on t USING HASH (i);
> > DELETE FROM t WHERE i = 1;
> > DELETE FROM t WHERE i = 1;
> > 
> > triggers
> > TRAP: failed Assert("BufferIsValid(buffer)"), File: "bufmgr.c", Line: 497, PID: 3942058
> > 
> > #4  0x000079a60ae288ff in __GI_abort () at ./stdlib/abort.c:79
> > #5  0x00005a68d9343eef in ExceptionalCondition (conditionName=conditionName@entry=0x5a68d93ac27d "BufferIsValid(buffer)",
> >     fileName=fileName@entry=0x5a68d93c99ef "bufmgr.c", lineNumber=lineNumber@entry=497) at assert.c:65
> > #6  0x00005a68d91a18eb in GetPrivateRefCountEntry (do_move=true, buffer=<optimized out>) at bufmgr.c:497
> > #7  SharedBufferBeginSetHintBits (lockstate=<synthetic pointer>, buf_hdr=0x79e5febbbc40, buffer=<optimized out>)
> >     at bufmgr.c:6830
> > #8  BufferBeginSetHintBits (buffer=<optimized out>) at bufmgr.c:6931
> > #9  0x00005a68d8e3c862 in _hash_kill_items (scan=<optimized out>) at hashutil.c:603
> > #10 0x00005a68d8e3b7c3 in _hash_next (scan=0x5a68e735f938, dir=<optimized out>) at hashsearch.c:69
> > #11 0x00005a68d8e616ce in index_getnext_tid (scan=scan@entry=0x5a68e735f938, direction=direction@entry=ForwardScanDirection)
> >     at indexam.c:647
> > ...
> > #25 0x00005a68d91eb4ad in exec_simple_query (query_string=0x5a68e7270120 "DELETE FROM t WHERE i = 1;") at postgres.c:1277
> > ...
> > 
> > Could you please look at this?
> 
> Yea, it's a stupid small mistake. Alexander Kuzmenkov reported it late
> afternoon yesterday, privately as I just noticed, and I was too tired to make
> sure an added test wouldn't have stability issues.
> 
> Will fix in the next few hours.

Took longer, sorry.  But it's pushed now.

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