linux-kernel - Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

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Message-ID: <20070810150213.GB8511@linux.vnet.ibm.com>
Date:	Fri, 10 Aug 2007 08:02:13 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, csnook@...hat.com,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org, heiko.carstens@...ibm.com,
	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
Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

On Fri, Aug 10, 2007 at 11:08:20AM +0200, Andi Kleen wrote:
> On Friday 10 August 2007 10:21:46 Herbert Xu wrote:
> > Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> > >
> > > The compiler is within its rights to read a 32-bit quantity 16 bits at
> > > at time, even on a 32-bit machine.  I would be glad to help pummel any
> > > compiler writer that pulls such a dirty trick, but the C standard really
> > > does permit this.
> > 
> > Code all over the kernel assumes that 32-bit reads/writes
> > are atomic so while such a compiler might be legal it certainly
> > can't compile Linux.
> 
> Yes, the kernel requirements are much stricter than ISO-C. And besides
> it is a heavy user of C extensions anyways. On the other hand some of the
> C99 extensions are not allowed. And then there is sparse, which enforces
> a language which sometimes is quite far from standard C. You could say it is 
> written in Linux-C, not ISO C. 

Understood.  My question is "why do we want the semantics of atomic_read()
and atomic_set() to differ?"

						Thanx, Paul
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