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Date: Thu, 22 Dec 2005 17:44:15 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Nicolas Pitre <nico@cam.org>
Cc: Christoph Hellwig <hch@infradead.org>, lkml <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@osdl.org>, Andrew Morton <akpm@osdl.org>,
       Arjan van de Ven <arjanv@infradead.org>,
       Jes Sorensen <jes@trained-monkey.org>,
       Zwane Mwaikambo <zwane@arm.linux.org.uk>,
       Oleg Nesterov <oleg@tv-sign.ru>, David Howells <dhowells@redhat.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>, Benjamin LaHaise <bcrl@kvack.org>,
       Steven Rostedt <rostedt@goodmis.org>, Andi Kleen <ak@suse.de>,
       Russell King <rmk+lkml@arm.linux.org.uk>
Subject: Re: [patch 0/9] mutex subsystem, -V4
Message-ID: <20051222164415.GA10628@elte.hu>
References: <20051222114147.GA18878@elte.hu> <20051222115329.GA30964@infradead.org> <Pine.LNX.4.64.0512221025070.26663@localhost.localdomain> <20051222154012.GA6284@elte.hu> <Pine.LNX.4.64.0512221113560.26663@localhost.localdomain>
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* Nicolas Pitre <nico@cam.org> wrote:

> > i'm curious, how would this ARMv6 solution look like, and what would be 
> > the advantages over the atomic swap based variant?
> 
> On ARMv6 (which can be SMP) the atomic swap instruction is much more 
> costly than on former ARM versions.  It however has ll/sc instructions 
> which allows it to implement a true atomic decrement, and the lock 
> fast path would look like: [...]

but couldnt you implement atomic_dec_return() with the ll/sc 
instructions? Something like:

repeat:
       ldrex   r1, [r0]
       sub     r1, r1, #1
       strex   r2, r1, [r0]
       orrs    r0, r2, r1
       jneq    repeat

(shot-in-the-dark guess at ARMv6 assembly)

hm?

	Ingo