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From: Denys Vlasenko <vda.linux@googlemail.com>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
Date: Mon, 10 Sep 2007 15:38:23 +0100
References: <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <200709101156.30010.vda.linux@googlemail.com> <20070910155156.2c1453fc@laptopd505.fenrus.org>
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To: Arjan van de Ven <arjan@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, Nick Piggin <piggin@cyberone.com.au>, Satyam Sharma <satyam@infradead.org>, Herbert Xu <herbert@gondor.apana.org.au>, Paul Mackerras <paulus@samba.org>, Christoph Lameter <clameter@sgi.com>, Chris Snook <csnook@redhat.com>, Ilpo Jarvinen <ilpo.jarvinen@helsinki.fi>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Stefan Richter <stefanr@s5r6.in-berlin.de>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch@vger.kernel.org, Netdev <netdev@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, ak@suse.de, heiko.carstens@de.ibm.com, David Miller <davem@davemloft.net>, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org
List-ID: <linux-arch.vger.kernel.org>

On Monday 10 September 2007 15:51, Arjan van de Ven wrote:
> On Mon, 10 Sep 2007 11:56:29 +0100
> Denys Vlasenko <vda.linux@googlemail.com> wrote:
> 
> > 
> > Well, if you insist on having it again:
> > 
> > Waiting for atomic value to be zero:
> > 
> >         while (atomic_read(&x))
> >                 continue;
> > 
> 
> and this I would say is buggy code all the way.
>
> Not from a pure C level semantics, but from a "busy waiting is buggy"
> semantics level and a "I'm inventing my own locking" semantics level.

After inspecting arch/*, I cannot agree with you.
Otherwise almost all major architectures use
"conceptually buggy busy-waiting":

arch/alpha
arch/i386
arch/ia64
arch/m32r
arch/mips
arch/parisc
arch/powerpc
arch/sh
arch/sparc64
arch/um
arch/x86_64

All of the above contain busy-waiting on atomic_read.

Including these loops without barriers:

arch/mips/kernel/smtc.c
			while (atomic_read(&idle_hook_initialized) < 1000)
				;
arch/mips/sgi-ip27/ip27-nmi.c
	while (atomic_read(&nmied_cpus) != num_online_cpus());

[Well maybe num_online_cpus() is a barrier, I didn't check]

arch/sh/kernel/smp.c
	if (wait)
		while (atomic_read(&smp_fn_call.finished) != (nr_cpus - 1));

Bugs?
--
vda