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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [Q] Implementation of spin_lock on i386: why "rep;nop" ?
Date: 17 Sep 2001 12:47:27 -0700
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In-Reply-To: <Pine.LNX.4.31.0109171725140.26090-100000@sisley.ri.silicomp.fr> <E15j2BM-0007WU-00@the-village.bc.nu>
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Copyright: Copyright 2001 H. Peter Anvin - All Rights Reserved
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Followup to:  <E15j2BM-0007WU-00@the-village.bc.nu>
By author:    Alan Cox <alan@lxorguk.ukuu.org.uk>
In newsgroup: linux.dev.kernel
>
> > The "rep;nop" line looks dubious, since the IA-32 programmer's manual from
> > Intel (year 2001) mentions that the behaviour of REP is undefined when it
> > is not used with string opcodes. BTW, according to the same manual, REP is
> > supposed to modify ecx, but it looks like is is not the case here... which
> > is fortunate, since ecx is never saved. :-)
> 
> rep nop is a pentium IV operation. Its retroactively after testing defined
> to be portable and ok. 
> 

Now, the example brought up was assembly, but in general I really
think we should have a processor-independent wait_loop(); inline.
Right now we have a rep_nop(); inline which only works on x86 (and
presumably x86-64).

	-hpa
-- 
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"Unix gives you enough rope to shoot yourself in the foot."
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