linux-kernel - HAVE_CMPXCHG_LOCAL arch support.

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Date:	Wed, 10 Jun 2015 17:57:09 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Xose Vazquez Perez <xose.vazquez@...il.com>
Cc:	linux-arch@...r.kernel.org, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: HAVE_CMPXCHG_LOCAL arch support.

On Wed, Jun 10, 2015 at 04:58:04PM +0200, Ralf Baechle wrote:

> On Mon, Jun 08, 2015 at 03:14:43PM +0200, Xose Vazquez Perez wrote:
> 
> > If there is anything wrong, please report it in this thread:
> > https://marc.info/?t=143332955700003
> 
> 
> >    locking/ cmpxchg-local        : TODO |                  HAVE_CMPXCHG_LOCAL #  arch supports the this_cpu_cmpxchg() API
> 
> This one was easy - we have the functions in the code just no "select
> HAVE_CMPXCHG_LOCAL" Kconfig.

Something's wrong there.  The new file
Documentation/features/locking/cmpxchg-local/arch-support.txt in linux-next
claims correctly that only s390 and x86 define HAVE_CMPXCHG_LOCAL.  And a
git grep -w cmpxchg_local finds that in addition to these alpha, arm, arm64,
avr32, blackfin, c6x, frv, ia64, m32r, m68k, mips, parisc, powerpc, sparc,
unicore32 and xtensa define cmpxchg_local.

These architectures seem to not define cmpxchg_local in their arch/ dir:

  arc cris hexagon metag microblaze mn10300 nios2 openrisc score sh tile um

Microblaze and nios2 include <asm-generic/cmpxchg.h> into their arch
cmpxchg.h so they get a definition of these functions but don't define
HAVE_CMPXCHG_LOCAL.  Peter Zijlstra said it the local versions are ~ 20
cycles faster on x86 than the "global" version.  But I've found one user
of cmpxchg_local, mm/vmstat.c and one user of cmpxchg64_local,
drivers/iommu/intel-iommu.c.  Sure, fixing the issue was trivial for me
on MIPS but is having cmpxchg{,64}_local actually worth it?  

Cheers,

  Ralf
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