linux-kernel - Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic

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Date:	Mon, 17 May 2010 10:58:34 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	willy@...ux.intel.com, benh@...nel.crashing.org, paulus@...ba.org,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 2/2]: atomic_t: Remove volatile from atomic_t definition

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!
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