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

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Message-ID: <46C4ABA5.9010804@redhat.com>
Date:	Thu, 16 Aug 2007 15:55:17 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....com>,
	"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,
	Linus Torvalds <torvalds@...ux-foundation.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

Ilpo Järvinen wrote:
> On Thu, 16 Aug 2007, Herbert Xu wrote:
> 
>> We've been through that already.  If it's a busy-wait it
>> should use cpu_relax. 
> 
> I looked around a bit by using some command lines and ended up wondering 
> if these are equal to busy-wait case (and should be fixed) or not:
> 
> ./drivers/telephony/ixj.c
> 6674:   while (atomic_read(&j->DSPWrite) > 0)
> 6675-           atomic_dec(&j->DSPWrite);
> 
> ...besides that, there are couple of more similar cases in the same file 
> (with braces)...

atomic_dec() already has volatile behavior everywhere, so this is 
semantically okay, but this code (and any like it) should be calling 
cpu_relax() each iteration through the loop, unless there's a compelling 
reason not to.  I'll allow that for some hardware drivers (possibly this 
one) such a compelling reason may exist, but hardware-independent core 
subsystems probably have no excuse.

If the maintainer of this code doesn't see a compelling reason to add 
cpu_relax() in this loop, then it should be patched.

	-- Chris
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