linux-kernel - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-ID: <20070821070837.GB32036@flint.arm.linux.org.uk>
Date:	Tue, 21 Aug 2007 08:08:37 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	Christoph Lameter <clameter@....com>,
	Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, linux-kernel@...r.kernel.org, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, rpjday@...dspring.com,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-arch@...r.kernel.org, jesper.juhl@...il.com,
	satyam@...radead.org, zlynx@....org, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Mon, Aug 20, 2007 at 05:05:18PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 21, 2007 at 01:02:01AM +0200, Segher Boessenkool wrote:
> > >>And no, RMW on MMIO isn't "problematic" at all, either.
> > >>
> > >>An RMW op is a read op, a modify op, and a write op, all rolled
> > >>into one opcode.  But three actual operations.
> > >
> > >Maybe for some CPUs, but not all.  ARM for instance can't use the
> > >load exclusive and store exclusive instructions to MMIO space.
> > 
> > Sure, your CPU doesn't have RMW instructions -- how to emulate
> > those if you don't have them is a totally different thing.
> 
> I thought that ARM's load exclusive and store exclusive instructions
> were its equivalent of LL and SC, which RISC machines typically use to
> build atomic sequences of instructions -- and which normally cannot be
> applied to MMIO space.

Absolutely correct.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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