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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: Yura Sokolov <y.sokolov@postgrespro.ru>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Noah Misch <noah@leadboat.com>, Heikki Linnakangas <hlinnaka@iki.fi>, 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: 2026-03-31T22:05:46Z
Lists: pgsql-hackers 
Hi,

On 2026-03-31 19:02:33 +0300, Yura Sokolov wrote:
> 27.03.2026 23:00, Andres Freund wrote:
> > On 2026-03-25 18:35:55 -0400, Andres Freund wrote:
> >> Running it through valgrind and then will work on reading through one more
> >> time and pushing them.
> > 
> > And done.
> > 
> > Phew, this project took way longer than I'd though it'd take.
> 
> In addition to bug with BM_IO_ERROR [1] , I found race condition in
> PinBuffer in this lines of code:
> 
> 	if (unlikely(skip_if_not_valid && !(old_buf_state & BM_VALID)))
> 		return false;
> 
> 	/*
> 	 * We're not allowed to increase the refcount while the buffer
> 	 * header spinlock is held. Wait for the lock to be released.
> 	 */
> 	if (old_buf_state & BM_LOCKED)
> 		old_buf_state = WaitBufHdrUnlocked(buf);
> 
> While we waited for buffer header for being unlocked, it may become
> invalid, isn't it?
> Therefore, check related to skip_if_not_valid have to happen after waiting.

Yea, that does seem wrong.  Not sure how it ended up that way.

I think it may be better to add a continue after the WaitBufHdrUnlocked(), so
that we restart the loop, rather than moving the skip_if_not_valid check.


> ....
> 
> Another question: previously we had to wait for buffer for being unlocked
> because UnlockBufHdr wrote to buf->state unconditionally, therefore our pin
> increment could be lost.
> Now UnlockBufHdr and UnlockBufHdrExt does proper atomic operations and
> preserves concurrent changes. Are we still need to wait?

Yes.


> Most of time PinBuffer is called protected by BufTable's partition LWLock,
> therefore buffer may not be changed in dramatic way.

I don't think the partition locks are sufficient protection for everything. We
have a few places in the code that want to be able to modify the buffer state
depending on whether the buffer is already pinned, and I don't think all of
them currently hold the relevant buffer mapping partition's lock.  If pinning
were not to wait for an existing header lock, such checks would not easily be
doable.

Perhaps we could fix all the relevant places by acquiring the partition lock
in a few more places. But I think that'd be going in exactly the opposite
direction we should to. The partition locks are quite contended locks and we
should work on getting rid of them eventually. Building them into the
protection model seems quite unwise.


I think many of the places that currently do rely on the buffer header
spinlock can be converted to CAS loops.

I'm not really sure how much that's worth though - the WaitBufHdrLocked() in
PinBuffer() is pretty hard to hit in realistic workloads. What would be really
nice, is to be able to replace the CAS() with an atomic add (since those are
considerably faster), but that's not really possible regardless of the need
for WaitBufHdrLocked(), because we can't just add BUF_USAGECOUNT_ONE, as that
would allow increasing the usage count too far.

I would like to eventually narrow the definition of the buffer header spinlock
to just be about the "identity" of the buffer, which then would only be needed
by things like DropRelationBuffers() and when changing the buffer's identity.


> But call in ReadRecentBuffer is the exception. It is not protected by
> partition lock and have to make additional checks. That is why you
> introduced skip_if_not_valid.
> 
> Does optimization of ReadRecentBuffer pays for WaitBufHdrUnlocked?

As mentioned above, I don't think it's just ReadRecentBuffer that relies on
the buffer header spinlock preventing new pins.

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