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

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Date:	Mon, 17 Feb 2014 16:18:52 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Torvald Riegel <triegel@...hat.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.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 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.

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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