linux-kernel - Re: [RFC][PATCH 0/5] arch: atomic rework

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Date:	Mon, 17 Feb 2014 17:26:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Torvald Riegel <triegel@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
	David Howells <dhowells@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework

On Mon, Feb 17, 2014 at 04:18:52PM -0800, Linus Torvalds wrote:
> On Mon, Feb 17, 2014 at 3:41 PM, Torvald Riegel <triegel@...hat.com> wrote:
> >
> > There's an underlying problem here that's independent from the actual
> > instance that you're worried about here: "no sense" is a ultimately a
> > matter of taste/objectives/priorities as long as the respective
> > specification is logically consistent.
> 
> Yes. But I don't think it's "independent".
> 
> Exactly *because* some people will read standards without applying
> "does the resulting code generation actually make sense for the
> programmer that wrote the code", the standard has to be pretty clear.
> 
> The standard often *isn't* pretty clear. It wasn't clear enough when
> it came to "volatile", and yet that was a *much* simpler concept than
> atomic accesses and memory ordering.
> 
> And most of the time it's not a big deal. But because the C standard
> generally tries to be very portable, and cover different machines,
> there tends to be a mindset that anything inherently unportable is
> "undefined" or "implementation defined", and then the compiler writer
> is basically given free reign to do anything they want (with
> "implementation defined" at least requiring that it is reliably the
> same thing).
> 
> And when it comes to memory ordering, *everything* is basically
> non-portable, because different CPU's very much have different rules.
> I worry that that means that the standard then takes the stance that
> "well, compiler re-ordering is no worse than CPU re-ordering, so we
> let the compiler do anything". And then we have to either add
> "volatile" to make sure the compiler doesn't do that, or use an overly
> strict memory model at the compiler level that makes it all pointless.

For whatever it is worth, this line of reasoning has been one reason why
I have been objecting strenuously every time someone on the committee
suggests eliminating "volatile" from the standard.

							Thanx, Paul

> So I really really hope that the standard doesn't give compiler
> writers free hands to do anything that they can prove is "equivalent"
> in the virtual C machine model. That's not how you get reliable
> results.
> 
>                Linus
> 

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