Horton

Re: [HACKERS] Deadlock in XLogInsert at AIX

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Noah Misch <noah@leadboat.com>
Cc: Michael Paquier <michael.paquier@gmail.com>, Andres Freund <andres@anarazel.de>, Heikki Linnakangas <hlinnaka@iki.fi>, Konstantin Knizhnik <k.knizhnik@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Bernd Helmle <mailings@oopsware.de>
Date: 2019-10-07T19:06:35Z
Lists: pgsql-hackers 
Noah Misch <noah@leadboat.com> writes:
> [ fetch-add-gcc-xlc-unify-v2.patch ]

This still fails on Apple's compilers.  The first failure I get is

ccache gcc -std=gnu99 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -g -O2 -I../../../src/include -I/usr/local/include -isysroot /Developer/SDKs/MacOSX10.5.sdk    -c -o nodeHashjoin.o nodeHashjoin.c
/var/tmp//ccXUM8ep.s:449:Parameter error: r0 not allowed for parameter 2 (code as 0 not r0)

Line 449 of the assembly file is the addi in

LM87:
                sync
        lwarx   r0,0,r2
        addi    r11,r0,1
        stwcx.  r11,0,r2
        bne             $-12
        isync

which I suppose comes out of PG_PPC_FETCH_ADD.  I find this idea of
constructing assembly code by string-pasting pretty unreadable and am not
tempted to try to debug it, but I don't immediately see why this doesn't
work when the existing s_lock.h code does.  I think that the assembler
error message is probably misleading: while it seems to be saying to
s/r0/0/ in the addi, gcc consistently uses "rN" syntax for the second
parameter elsewhere.  I do note that gcc never generates r0 as addi's
second parameter in several files I checked through, so maybe what it
means is "you need to use some other register"?  (Which would imply that
the constraint for this asm argument is too loose.)

I'm also wondering why this isn't following s_lock.h's lead as to
USE_PPC_LWARX_MUTEX_HINT and USE_PPC_LWSYNC.

			regards, tom lane

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.