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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: Antonin Houska <ah@cybertec.at>
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: 2026-03-11T23:09:26Z
Lists: pgsql-hackers 
Hi,

On 2026-02-12 11:36:08 +0100, Antonin Houska wrote:
> Andres Freund <andres@anarazel.de> wrote:
>
> > For something committable, I think we should probably split IsMVCCSnapshot
> > into IsMVCCSnapshot(), just accepting SNAPSHOT_MVCC, and IsMVCCLikeSnapshot()
> > accepting both SNAPSHOT_MVCC and SNAPSHOT_HISTORIC_MVCC. And then go through
> > all the existing callers of IsMVCCSnapshot() - only about half should stay
> > as-is, I think.
>
> The attached patch tries to do that.

Thanks!


> From dcdbaf3095e632a1f7f65f3abc43eccff0249d4c Mon Sep 17 00:00:00 2001
> From: Antonin Houska <ah@cybertec.at>
> Date: Thu, 12 Feb 2026 11:14:00 +0100
> Subject: [PATCH] Refine checking of snapshot type.
>
> It appears to be confusing if IsMVCCSnapshot() evaluates to true for both
> "regular" and "historic" MVCC snapshot. This patch restricts the meaning of
> the macro to the "regular" MVCC snapshot, and introduces a new macro
> IsMVCCLikeSnapshot() to recognize both types.
>
> IsMVCCLikeSnapshot() is only used in functions that can (supposedly) be called
> during logical decoding.

I think I agree with where you selected IsMVCCSnapshot() and where you
selected IsMVCCLikeSnapshot().


> diff --git a/src/include/utils/snapmgr.h b/src/include/utils/snapmgr.h
> index b8c01a291a1..dd5aaae6953 100644
> --- a/src/include/utils/snapmgr.h
> +++ b/src/include/utils/snapmgr.h
> @@ -53,12 +53,14 @@ extern PGDLLIMPORT SnapshotData SnapshotToastData;
>
>  /* This macro encodes the knowledge of which snapshots are MVCC-safe */
>  #define IsMVCCSnapshot(snapshot)  \
> -	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
> -	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
> +	((snapshot)->snapshot_type == SNAPSHOT_MVCC)
>
>  #define IsHistoricMVCCSnapshot(snapshot)  \
>  	((snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
>
> +#define IsMVCCLikeSnapshot(snapshot)  \
> +	(IsMVCCSnapshot(snapshot) || IsHistoricMVCCSnapshot(snapshot))
> +
>  extern Snapshot GetTransactionSnapshot(void);
>  extern Snapshot GetLatestSnapshot(void);
>  extern void SnapshotSetCommandId(CommandId curcid);

Probably need to update the comments a bit.  What about something like


/*
 * Is the snapshot implemented as an MVCC snapshot (i.e. it uses
 * SNAPSHOT_MVCC).  If so, there will be at most be one visible row in a chain
 * of updated tuples, and each visible tuple will be seen exactly once.
 */
#define IsMVCCSnapshot(snapshot)  \
...

/*
 * Is the snapshot either an MVCC snapshot or has equivalent visibility
 * semantics (see IsMVCCSnapshot()).
 */
#define IsMVCCLikeSnapshot(snapshot)  \


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