Horton

Re: [HACKERS] Deadlock in XLogInsert at AIX

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Michael Paquier <michael.paquier@gmail.com>, Andres Freund <andres@anarazel.de>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Konstantin Knizhnik <k.knizhnik@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Bernd Helmle <mailings@oopsware.de>
Date: 2018-01-17T08:36:31Z
Lists: pgsql-hackers 
On Tue, Jan 16, 2018 at 08:50:24AM -0800, Andres Freund wrote:
> On 2018-01-16 16:12:11 +0900, Michael Paquier wrote:
> > On Fri, Feb 03, 2017 at 12:26:50AM +0000, Noah Misch wrote:
> > > Since this emits double syncs with older xlc, I recommend instead replacing
> > > the whole thing with inline asm.  As I opined in the last message of the
> > > thread you linked above, the intrinsics provide little value as abstractions
> > > if one checks the generated code to deduce how to use them.  Now that the
> > > generated code is xlc-version-dependent, the port is better off with
> > > compiler-independent asm like we have for ppc in s_lock.h.
> > 
> > Could it be cleaner to just use __xlc_ver__ to avoid double syncs on
> > past versions? I think that it would make the code more understandable
> > than just listing directly the instructions.
> 
> Given the quality of the intrinsics on AIX, see past commits and the
> comment in the code quoted above, I think we're much better of doing
> this via inline asm.

For me, verifiability is the crucial benefit of inline asm.  Anyone with an
architecture manual can thoroughly review an inline asm implementation.  Given
intrinsics and __xlc_ver__ conditionals, the same level of review requires
access to every xlc version.

> > As there have been other
> > bug reports from Tony Reix who has been working on AIX with XLC 13.1 and
> > that this thread got lost in the wild, I have added an entry in the next
> > CF:
> > https://commitfest.postgresql.org/17/1484/

The most recent patch version is Returned with Feedback.  As a matter of
procedure, I discourage creating commitfest entries as a tool to solicit new
patch versions.  If I were the author of a RwF patch, I would dislike finding
a commitfest entry that I did not create with myself listed as author.

If you do choose to proceed, the entry should be Waiting on Author.

> > As Heikki is not around these days, Noah, could you provide a new
> > version of the patch? This bug has been around for some time now, it
> > would be nice to move on.. 

Not soon.

Note that fixing this bug is just the start of accepting XLC 13.1 as a
compiler of PostgreSQL.  If we get a buildfarm member with a few dozen clean
runs (blocked by, at a minimum, fixing this and the inlining bug), we'll have
something.  Until then, support for XLC 13.1 is an anti-feature.

nm

Commits

  1. For all ppc compilers, implement compare_exchange and fetch_add with asm.

  2. For PowerPC instruction "addi", use constraint "b".

  3. For all ppc compilers, implement pg_atomic_fetch_add_ with inline asm.

  4. Replace xlc __fetch_and_add() with inline asm.

  5. Test pg_atomic_fetch_add_ with variable addend and 16-bit edge cases.