From mboxrd@z Thu Jan  1 00:00:00 1970
From: Heiko Carstens <heiko.carstens@de.ibm.com>
Subject: Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition
Date: Mon, 17 May 2010 10:58:34 +0200
Message-ID: <20100517085833.GA2404@osiris.boeblingen.de.ibm.com>
References: <20100517043353.GA22127@kryten>
 <20100517043457.GA22416@kryten>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-arch-owner@vger.kernel.org>
Received: from mtagate2.uk.ibm.com ([194.196.100.162]:46833 "EHLO
	mtagate2.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751830Ab0EQI5l (ORCPT
	<rfc822;linux-arch@vger.kernel.org>); Mon, 17 May 2010 04:57:41 -0400
Content-Disposition: inline
In-Reply-To: <20100517043457.GA22416@kryten>
Sender: linux-arch-owner@vger.kernel.org
List-ID: <linux-arch.vger.kernel.org>
To: Anton Blanchard <anton@samba.org>
Cc: torvalds@linux-foundation.org, akpm@linux-foundation.org, willy@linux.intel.com, benh@kernel.crashing.org, paulus@samba.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Martin Schwidefsky <schwidefsky@de.ibm.com>

On Mon, May 17, 2010 at 02:34:57PM +1000, Anton Blanchard wrote:
> 
> When looking at a performance problem on PowerPC, I noticed some awful code
> generation:
> 
> c00000000051fc98:       3b 60 00 01     li      r27,1
> ...
> c00000000051fca0:       3b 80 00 00     li      r28,0
> ...
> c00000000051fcdc:       93 61 00 70     stw     r27,112(r1)
> c00000000051fce0:       93 81 00 74     stw     r28,116(r1)
> c00000000051fce4:       81 21 00 70     lwz     r9,112(r1)
> c00000000051fce8:       80 01 00 74     lwz     r0,116(r1)
> c00000000051fcec:       7d 29 07 b4     extsw   r9,r9
> c00000000051fcf0:       7c 00 07 b4     extsw   r0,r0
> 
> c00000000051fcf4:       7c 20 04 ac     lwsync
> c00000000051fcf8:       7d 60 f8 28     lwarx   r11,0,r31
> c00000000051fcfc:       7c 0b 48 00     cmpw    r11,r9
> c00000000051fd00:       40 c2 00 10     bne-    c00000000051fd10
> c00000000051fd04:       7c 00 f9 2d     stwcx.  r0,0,r31
> c00000000051fd08:       40 c2 ff f0     bne+    c00000000051fcf8
> c00000000051fd0c:       4c 00 01 2c     isync
> 
> We create two constants, write them out to the stack, read them straight back
> in and sign extend them. What a mess.
> 
> It turns out this bad code is a result of us defining atomic_t as a
> volatile int.
> 
> We removed the volatile attribute from the powerpc atomic_t definition years
> ago, but commit ea435467500612636f8f4fb639ff6e76b2496e4b (atomic_t: unify all
> arch definitions) added it back in. 

With your patches we can also revert 39475179d40996b4efa662e3825735a84d2526d1
"[S390] Improve code generated by atomic operations." which was created for
the very same reason.

Thanks!