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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: Chao Li <li.evan.chao@gmail.com>
Cc: 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: 2026-01-14T16:23:05Z
Lists: pgsql-hackers 
Hi,

On 2026-01-14 10:26:07 +0800, Chao Li wrote:
> So far I’ve only reviewed 0001 and 0002. I’m not very familiar with this area, so the review has been a bit slow.
> 
> Overall, 0001 looks good to me. It renames LW_FLAG_RELEASE_OK to
> LW_FLAG_WAKE_IN_PROGRESS and inverts the meaning, which makes sense. I only
> have a small nit on naming: the local variable “new_release_in_progress". I
> see that it’s inherited from the old name and was updated from “_ok" to
> “_in_progress", but now that the flag itself is renamed, would it make sense
> to rename the variable as well? Something like “wake_in_progress" or
> “new_wake_in_progress" might better reflect the new flag name.

Agreed that is better. Updated that way.



> In 0002, a bunch of new macros are introduced. My initial impression wasn’t
> great, mostly due to the amount of line wrapping.

I think the previous formatting made it hard to actually write useful comments
and caused line-length problems in the subsequent commits. Lines are cheap.


> Looking a bit closer, I also noticed some duplication, for example,
> "BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS" appears more than once

Yea, that's probably better to avoid. I'll add a fix to that in the commit
changing it to 64bits, I think.


> ; and a small inconsistency between BUF_STATE_GET_REFCOUNT and
> BUF_STATE_GET_USAGECOUNT (even though the former doesn’t actually need a
> shift).

I don't see the point, if we later want to move refcounts elsewhere, we can do
it at that time.


> I tried a small refactor of the macro definitions in the attached diff to
> see if things could be made a bit more regular. It introduces a helper macro
> MASK() and a BUF_REFCOUNT_SHIFT constant, and removes a bit of
> duplication. If you like it, feel free to take it; otherwise, please just
> ignore it. Note that, the diff is based on 0002.

I don't think the MASK thing is an improvement.


> (I actually hesitated to attach a diff, because if you’ve already created a
> CF entry, the attached diff could break the CI tests. If that happens, sorry
> about that.)

FWIW, there's a trick to avoid that: Rename your patch to end in .txt or such.


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