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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>
Cc: 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: 2026-01-15T23:16:07Z
Lists: pgsql-hackers 
Hi,

On 2026-01-15 18:02:34 -0500, Tom Lane wrote:
> In file included from ../pgsql/src/include/pgstat.h:24,
>                  from ../pgsql/src/backend/storage/buffer/bufmgr.c:52:
> In function \342\200\230pgstat_report_wait_start\342\200\231,
>     inlined from \342\200\230BufferLockAcquire\342\200\231 at ../pgsql/src/backend/storage/buffer/bufmgr.c:5833:3:
> ../pgsql/src/include/utils/wait_event.h:75:49: warning: \342\200\230wait_event\342\200\231 may be used uninitialized [-Wmaybe-uninitialized]
>    75 |         *(volatile uint32 *) my_wait_event_info = wait_event_info;
> [...]
> Apparently they do not find the coding in this switch persuasive:
> 
> 		switch (mode)
> 		{
> 			case BUFFER_LOCK_EXCLUSIVE:
> 				wait_event = WAIT_EVENT_BUFFER_EXCLUSIVE;
> 				break;
> 			case BUFFER_LOCK_SHARE_EXCLUSIVE:
> 				wait_event = WAIT_EVENT_BUFFER_SHARE_EXCLUSIVE;
> 				break;
> 			case BUFFER_LOCK_SHARE:
> 				wait_event = WAIT_EVENT_BUFFER_SHARED;
> 				break;
> 			case BUFFER_LOCK_UNLOCK:
> 				pg_unreachable();
> 
> 		}
> 
> It's not clear to me whether that's more about not believing
> pg_unreachable() or more about the lack of a default: case.

> I see that this is a modification of code that existed before
> fcb9c977a and wasn't being complained of, which makes it even
> stranger.

The code before did have a default:, so I guess that could be the
difference...

I can reproduce it here if I build with -Og or -O1, but not with -O2, so I
guess it's "just" the optimizer falling short somehow.

Huh, interesting, the optimization level dependency made me curious. Turns out
the warning goes away if I actually force the inlining of BufferLockAcquire()
with pg_attribute_always_inline. Presumably because then the compiler realizes
that the mode argument is always a constant that it can evaluate.

Can't quite decide between just using always_inline - after all the buffer
locking really is a rather crucial code path - and just initializing
wait_event to 0. Either seems better than using a default:.

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