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Message-Id: <200709101538.25132.vda.linux@googlemail.com>
Date: Mon, 10 Sep 2007 15:38:23 +0100
From: Denys Vlasenko <vda.linux@...glemail.com>
To: Arjan van de Ven <arjan@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Nick Piggin <piggin@...erone.com.au>,
Satyam Sharma <satyam@...radead.org>,
Herbert Xu <herbert@...dor.apana.org.au>,
Paul Mackerras <paulus@...ba.org>,
Christoph Lameter <clameter@....com>,
Chris Snook <csnook@...hat.com>,
Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Stefan Richter <stefanr@...6.in-berlin.de>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-arch@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>, ak@...e.de,
heiko.carstens@...ibm.com, David Miller <davem@...emloft.net>,
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rpjday@...dspring.com, jesper.juhl@...il.com,
segher@...nel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
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@...glemail.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
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