linux-kernel - Re: [PATCH 1/4] arch: local64

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Message-ID: <23895.1274453549@redhat.com>
Date:	Fri, 21 May 2010 15:52:29 +0100
From:	David Howells <dhowells@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	dhowells@...hat.com, Ingo Molnar <mingo@...e.hu>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>,
	Paul Mundt <lethal@...ux-sh.org>,
	Will Deacon <will.deacon@....com>,
	Deng-Cheng Zhu <dengcheng.zhu@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH 1/4] arch: local64_t



Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> + * This is the default implementation, which uses atomic64_t.  Which is
> + * rather pointless.  The whole point behind local64_t is that some processors
> + * can perform atomic adds and subtracts in a manner which is atomic wrt IRQs
> + * running on this CPU.  local64_t allows exploitation of such capabilities.

Interesting...  What FRV does in atomic64-ops.S should probably be rebranded
local64_t, and atomic64_t ops be based on that in non-SMP mode.

What I did on FRV was to emulate LL/ST instructions using some of the
excessive numbers of conditional bits to do so - but it only works on UP
systems.

David
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