Re: [HACKERS] Deadlock in XLogInsert at AIX

On 01.02.2017 15:39, Heikki Linnakangas wrote:

On 02/01/2017 01:07 PM, Konstantin Knizhnik wrote:

Attached please find my patch for XLC/AIX.
The most critical fix is adding __sync to pg_atomic_fetch_add_u32_impl.
The comment in this file says that:

       * __fetch_and_add() emits a leading "sync" and trailing "isync",
thereby
       * providing sequential consistency.  This is undocumented.

But it is not true any more (I checked generated assembler code in
debugger).
This is why I have added __sync() to this function. Now pgbench working
normally.

Seems like it was not so much undocumented, but an implementation detail that was not guaranteed after all..

Does __fetch_and_add emit a trailing isync there either? Seems odd if __compare_and_swap requires it, but __fetch_and_add does not. Unless we can find conclusive documentation on that, I think we should assume that an __isync() is required, too.

There was a long thread on these things the last time this was changed: https://www.postgresql.org/message-id/20160425185204.jrvlghn3jxulsb7i%40alap3.anarazel.de. I couldn't find an explanation there of why we thought that fetch_and_add implicitly performs sync and isync.

Also there is mysterious disappearance of assembler section function
with sync instruction from pg_atomic_compare_exchange_u32_impl.
I have fixed it by using __sync() built-in function instead.

__sync() seems more appropriate there, anyway. We're using intrinsics for all the other things in generic-xlc.h. But it sure is scary that the "asm" sections just disappeared.

In arch-ppc.h, shouldn't we have #ifdef __IBMC__ guards for the __sync() and __lwsync() intrinsics? Those are an xlc compiler-specific thing, right? Or if they are expected to work on any ppc compiler, then we should probably use them always, instead of the asm sections.

In summary, I came up with the attached. It's essentially your patch, with tweaks for the above-mentioned things. I don't have a powerpc system to test on, so there are probably some silly typos there.

Why do you prefer to use _check_lock instead of __check_lock_mp ?
First one is even not mentioned in XLC compiler manual:
http://www-01.ibm.com/support/docview.wss?uid=swg27046906&aid=7
or
http://scv.bu.edu/computation/bluegene/IBMdocs/compiler/xlc-8.0/html/compiler/ref/bif_sync.htm

- Heikki

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
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