linux-kernel - Re: [PATCH] make atomic_t volatile on all architectures

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Message-ID: <20070809083055.GA19048@gondor.apana.org.au>
Date:	Thu, 9 Aug 2007 16:30:55 +0800
From:	Herbert Xu <herbert.xu@...hat.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	ak@...e.de, heiko.carstens@...ibm.com, davem@...emloft.net,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH] make atomic_t volatile on all architectures

On Thu, Aug 09, 2007 at 03:47:57AM -0400, Chris Snook wrote:
>
> If they're not doing anything, sure.  Plenty of loops actually do some sort 
> of real work while waiting for their halt condition, possibly even work 
> which is necessary for their halt condition to occur, and you definitely 
> don't want to be doing cpu_relax() in this case.  On register-rich 
> architectures you can do quite a lot of work without needing to reuse the 
> register containing the result of the atomic_read().  Those are precisely 
> the architectures where barrier() hurts the most.

I have a problem with this argument.  The same loop could be
using a non-atomic as long as the updaters are serialised.  Would
you suggest that we turn such non-atomics into volatiles too?

Any loop that's waiting for an external halt condition either
has to schedule away (which is a barrier) or you'd be busy
waiting in which case you should use cpu_relax.

Do you have an example where this isn't the case?

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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