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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: Tom Lane <tgl@sss.pgh.pa.us>, Peter Geoghegan <pg@bowt.ie>
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: 2026-01-29T17:42:41Z
Lists: pgsql-hackers 
Hi,

On 2026-01-29 12:27:10 -0500, Andres Freund wrote:
> In the crashes due to Alexander's repro that I have looked at, the page is
> returned by SpGistNewBuffer()->GetFreeIndexPage(). Which afaict should only
> contain empty pages. It's easy to see how that could happen with concurrency,
> but there's none in the test.

I think I see how that happens - we reenter the page into the FSM during the
VACUUM that's part of the repro, but we previously also "recorded" it with
SpGistSetLastUsedPage(), presumably before it was emptied during vacuum.  The
we reuse the page first via the "last used page" cache, making it not
empty. Then we get the same page via the FSM, which wasn't updated when we
started filling the page via the last-used-page mechanism.


> I was just trying to repro this again while writing this message, and
> interestingly I got the same issue in nbtree this time. Which a) confirms
> Peter's statement that the "conditionally locking a buffer we already locked"
> issue exists for nbtree b) makes me suspect something odd is happening around
> indexfsm.

Not sure how that happens in a single threaded workload yet, though.


> diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
> index 6f935648ae9..f5602f4e7e1 100644
> --- a/src/backend/storage/buffer/bufmgr.c
> +++ b/src/backend/storage/buffer/bufmgr.c
> @@ -5895,6 +5895,13 @@ BufferLockUnlock(Buffer buffer, BufferDesc *buf_hdr)
>  
>  /*
>   * Acquire the content lock for the buffer, but only if we don't have to wait.
> + *
> + * It is allowed to try to conditionally acquire a lock on a buffer that this
> + * backend has already locked, but the lock acquisition will always fail, even
> + * if the new lock acquisition does not conflict with an already held lock
> + * (e.g. two share locks). This is because we don't track per-backend
> + * ownership of multiple lock levels.  That is ok for the current uses of
> + * BufferLockConditional().
>   */
>  static bool
>  BufferLockConditional(Buffer buffer, BufferDesc *buf_hdr, BufferLockMode mode)
> @@ -5902,11 +5909,16 @@ BufferLockConditional(Buffer buffer, BufferDesc *buf_hdr, BufferLockMode mode)
>  	PrivateRefCountEntry *entry = GetPrivateRefCountEntry(buffer, true);
>  	bool		mustwait;
>  
> -	/*
> -	 * We better not already hold a lock on the buffer.
> -	 */
>  	Assert(entry->data.lockmode == BUFFER_LOCK_UNLOCK);
>  

Err, this should have been removed, I accidentally re-added the hunk while
experimenting.

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