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Re: Buffer locking is special (hints, checksums, AIO writes)

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
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: 2026-01-15T23:02:34Z
Lists: pgsql-hackers 
Various buildfarm animals are complaining about fcb9c977a,
similarly to this from calliphoridae [1]:

[969/2355] ccache gcc -Isrc/backend/postgres_lib.a.p -Isrc/include -I../pgsql/src/include -I/usr/include/libxml2 -fdiagnostics-color=never -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -fno-strict-aliasing -fwrapv -fexcess-precision=standard -D_GNU_SOURCE -Wmissing-prototypes -Wpointer-arith -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security -Wdeclaration-after-statement -Wmissing-variable-declarations -Wno-format-truncation -Wno-stringop-truncation -O1 -ggdb -g3 -fno-omit-frame-pointer -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -DCOPY_PARSE_PLAN_TREES -DRAW_EXPRESSION_COVERAGE_TEST -fPIC -isystem /usr/include/mit-krb5 -pthread -DBUILDING_DLL -MD -MQ src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o -MF src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o.d -o src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o -c ../pgsql/src/backend/storage/buffer/bufmgr.c
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;
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
../pgsql/src/backend/storage/buffer/bufmgr.c: In function \342\200\230BufferLockAcquire\342\200\231:
../pgsql/src/backend/storage/buffer/bufmgr.c:5781:33: note: \342\200\230wait_event\342\200\231 was declared here
 5781 |                 uint32          wait_event;
      |                                 ^~~~~~~~~~

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.

			regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=calliphoridae&dt=2026-01-15%2019%3A35%3A00&stg=build

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