linux-kernel - RE: Linux 2.6.32-rc1

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Date:	Wed, 30 Sep 2009 17:42:02 -0700
From:	Yuhong Bao <yuhongbao_386@...mail.com>
To:	<arjan@...radead.org>, <eric.dumazet@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	<schwidefsky@...ibm.com>, <tglx@...utronix.de>,
	<johnstul@...ibm.com>, <linux-kernel@...r.kernel.org>
Subject: RE: Linux 2.6.32-rc1


 <20090930173102.2d54534c@...radead.org>
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----------------------------------------
> Date: Wed=2C 30 Sep 2009 17:31:02 +0200
> From: arjan@...radead.org
> To: eric.dumazet@...il.com
> CC: torvalds@...ux-foundation.org=3B schwidefsky@...ibm.com=3B tglx@...ut=
ronix.de=3B johnstul@...ibm.com=3B linux-kernel@...r.kernel.org=3B a.p.zijl=
stra@c
> Subject: Re: Linux 2.6.32-rc1
>
> On Wed=2C 30 Sep 2009 17:27:05 +0200
> Eric Dumazet  wrote:
>>
>>> + pop %edi
>> Why do you pop flags in edi=2C to later re-push them ?
>>
>>> + cli
>
> because here I disable interrupts
But popping the flags to edi to later repush them should not be necessary u=
nless you are switching esp.


Getting rid of this will allow the push/pop edi at the beginning/end to
be eliminated=2C reducing code size as well as increasing the speed.
> (basically this is local_irq_save() )
>>
>>
>>> + xchg (%esi)=2C %ebx
>>> + xchg 4(%esi)=2C %ecx
>> How this sequence is guaranteed to be atomic with other cpus ?
>
> it is not. this is the 486 implementation which is !SMP
> (just like the current cmpxchg64() fallback)



BTW=2C NT4 had a SMP version of this emulation that used a simple spinlock:
http://www.geoffchappell.com/viewer.htm?doc=3Dstudies/windows/km/cpu/cx8.ht=
m

Yuhong Bao
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