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

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Message-ID: <20070821161643.GA7574@linux.vnet.ibm.com>
Date:	Tue, 21 Aug 2007 09:16:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Russell King <rmk+lkml@....linux.org.uk>,
	Christoph Lameter <clameter@....com>,
	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, ak@...e.de, netdev@...r.kernel.org,
	cfriesen@...tel.com, akpm@...ux-foundation.org,
	rpjday@...dspring.com, Nick Piggin <nickpiggin@...oo.com.au>,
	linux-arch@...r.kernel.org, jesper.juhl@...il.com,
	satyam@...radead.org, zlynx@....org, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>, 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

On Tue, Aug 21, 2007 at 04:48:51PM +0200, Segher Boessenkool wrote:
> >>Let me say it more clearly: On ARM, it is impossible to perform atomic
> >>operations on MMIO space.
> >
> >Actually, no one is suggesting that we try to do that at all.
> >
> >The discussion about RMW ops on MMIO space started with a comment
> >attributed to the gcc developers that one reason why gcc on x86
> >doesn't use instructions that do RMW ops on volatile variables is that
> >volatile is used to mark MMIO addresses, and there was some
> >uncertainty about whether (non-atomic) RMW ops on x86 could be used on
> >MMIO.  This is in regard to the question about why gcc on x86 always
> >moves a volatile variable into a register before doing anything to it.
> 
> This question is GCC PR33102, which was incorrectly closed as a 
> duplicate
> of PR3506 -- and *that* PR was closed because its reporter seemed to
> claim the GCC generated code for an increment on a volatile (namely, 
> three
> machine instructions: load, modify, store) was incorrect, and it has to
> be one machine instruction.
> 
> >So the whole discussion is irrelevant to ARM, PowerPC and any other
> >architecture except x86[-64].
> 
> And even there, it's not something the kernel can take advantage of
> before GCC 4.4 is in widespread use, if then.  Let's move on.

I agree that instant gratification is hard to come by when synching
up compiler and kernel versions.  Nonetheless, it should be possible
to create APIs that are are conditioned on the compiler version.

						Thanx, Paul
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