Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>, Antonin Houska <ah@cybertec.at>
Date: 2025-05-09T01:06:18Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

Attachments

Hi,

On 2025-05-02 20:05:11 -0700, Noah Misch wrote:
> On Wed, Apr 30, 2025 at 04:00:35PM -0400, Andres Freund wrote:
> > pgaio_io_wait_for_free() does what it says on the tin. For that, after a bunch
> > of other things, finds the oldest in-flight IO and waits for it.
> > 
> > 		PgAioHandle *ioh = dclist_head_element(PgAioHandle, node,
> > 											   &pgaio_my_backend->in_flight_ios);
> > 
> > 		switch (ioh->state)
> > 		{
> > ...
> > 			case PGAIO_HS_COMPLETED_IO:
> > 			case PGAIO_HS_SUBMITTED:
> > 				pgaio_debug_io(DEBUG2, ioh,
> > 							   "waiting for free io with %d in flight",
> > 							   dclist_count(&pgaio_my_backend->in_flight_ios));
> > ...
> > 				pgaio_io_wait(ioh, ioh->generation);
> > 				break;
> > 
> > 
> > The problem is that, if the log level is low enough, ereport() (which is
> > called by pgaio_debug_io()), processes interrupts.  The interrupt processing
> > may end up execute ProcessBarrierSmgrRelease(), which in turn needs to wait
> > for all in-flight IOs before the IOs are closed.
> > 
> > Which then leads to the
> > 			elog(PANIC, "waiting for own IO in wrong state: %d",
> > 				 state);
> > 
> > error.
> 
> Printing state 0 (PGAIO_HS_IDLE), right?

Correct.


>I think the chief problem is that pgaio_io_wait_for_free() is fetching
>ioh->state, then possibly processing interrupts in pgaio_debug_io(), then
>finally fetching ioh->generation.  If it fetched ioh->generation to a local
>variable before pgaio_debug_io, I think that would resolve this one.

That's what I also concluded after playing around with a few different
approaches.


> Then the pgaio_io_was_recycled() would prevent the PANIC:
> 
> 	if (pgaio_io_was_recycled(ioh, ref_generation, &state))
> 		return;
> 
> 	if (am_owner)
> 	{
> 		if (state != PGAIO_HS_SUBMITTED
> 			&& state != PGAIO_HS_COMPLETED_IO
> 			&& state != PGAIO_HS_COMPLETED_SHARED
> 			&& state != PGAIO_HS_COMPLETED_LOCAL)
> 		{
> 			elog(PANIC, "waiting for own IO in wrong state: %d",
> 				 state);
> 		}
> 	}
> 
> Is that right?

Yes, it avoids the issue.


> If that's the solution, pgaio_closing_fd() and pgaio_shutdown() would need
> similar care around fetching the generation before the pgaio_debug_io.
> Maybe there's an opportunity for a common inline function.  Or at least a
> comment at the "generation" field on how to safely time a fetch thereof and
> any barrier required.

I didn't see a good way to move this into an inline function, unfortunately.


We do need to hold interrupts in a few other places, I think - with some debug
infrastructure (things like calling ProcessBarrierSmgrRelease() whenever
interrupts could be processed and calling CFI() in errstart() in its return
false case) it's possible to find state confusions which trigger
assertions. The issue is that pgaio_io_update_state() contains a
pgaio_debug_io() and executing pgaio_closing_fd() in places that call
pgaio_io_update_state() doesn't end well.  There's a similar danger with the
debug message in pgaio_io_reclaim().

In the attached patch I added an assertion to pgaio_io_update_state()
verifying that interrupts are held and added code to hold interrupts in the
relevant places.

> > I'm not yet sure how to best fix it - locally I have done so by pgaio_debug()
> > do a HOLD_INTERRUPTS()/RESUME_INTERRUPTS() around the call to ereport.  But
> > that doesn't really seem great - otoh requiring various pieces of code to know
> > that anything emitting debug messages needs to hold interrupts etc makes for
> > rare and hard to understand bugs.
> > 
> > We could just make the relevant functions hold interrupts, and that might be
> > the best path forward, but we don't really need to hold all interrupts
> > (e.g. termination would be fine), so it's a bit coarse grained.  It would need
> > to happen in a few places, which isn't great either.
> > 
> > Other suggestions?
> 
> For the "no free IOs despite no in-flight IOs" case, I'd replace the
> ereport(ERROR) with "return;" since we now know interrupt processing reclaimed
> an IO.

Hm - it seems better to me to check if there are now free handles and return
if that's the case, but to keep the error check in case there actually is no
free IO? That seems like a not implausible bug...


> Then decide what protection if any, we need against bugs causing an
> infinite loop in caller pgaio_io_acquire().  What's the case motivating the
> unbounded loop in pgaio_io_acquire(), as opposed to capping at two
> pgaio_io_acquire_nb() calls?  If the theory is that pgaio_io_acquire() could
> be reentrant, what scenario would reach that reentrancy?

I do not remember why I wrote this as an endless loop.  If you prefer I could
change that as part of this patch.


It does seem rather dangerous that errstart() processes interrupts for debug
messages, but only if the debug message is actually logged. That's really a
recipe for hard to find bugs.  I wonder if we should, at least in assertion
mode, process interrupts even if not emitting the message.

Greetings,

Andres Freund