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

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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>,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	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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