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

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Date:	Thu, 16 Aug 2007 06:28:42 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	clameter@....com, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>, davem@...emloft.net,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

[ Sorry for empty subject line in previous mail. I intended to make
  a patch so cleared it to change it, but ultimately neither made
  a patch nor restored subject line. Done that now. ]

On Thu, 16 Aug 2007, Herbert Xu wrote:

> On Thu, Aug 16, 2007 at 06:06:00AM +0530, Satyam Sharma wrote:
> > 
> > that are:
> > 
> > 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
> > 		mdelay(1);
> > 		msecs--;
> > 	}
> > 
> > where mdelay() becomes __const_udelay() which happens to be in another
> > translation unit (arch/i386/lib/delay.c) and hence saves this callsite
> > from being a bug :-)
> 
> The udelay itself certainly should have some form of cpu_relax in it.

Yes, a form of barrier() must be present in mdelay() or udelay() itself
as you say, having it in __const_udelay() is *not* enough (superflous
actually, considering it is already a separate translation unit and
invisible to the compiler).

However, there are no compiler barriers on the macro-definition-path
between mdelay(1) and __const_udelay(), so the only thing that saves us
from being a bug here is indeed the different-translation-unit concept.
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