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

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Message-ID: <CA+55aFwn9gXWVq_GL=tPPP63vsqs-9QB4ii4s06xqG4UscCV5w@mail.gmail.com>
Date:	Thu, 20 Feb 2014 10:32:51 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
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 Thu, Feb 20, 2014 at 10:11 AM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>
> You really need that "consume" to be "acquire".

So I think we now all agree that that is what the standard is saying.

And I'm saying that that is wrong, that the standard is badly written,
and should be fixed.

Because before the standard is fixed, I claim that "consume" is
unusable. We cannot trust it. End of story.

The fact that apparently gcc is currently buggy because it got the
dependency calculations *wrong* just reinforces my point.

The gcc bug Torvald pointed at is exactly because the current C
standard is illogical unreadable CRAP. I can guarantee that what
happened is:

 - the compiler saw that the result of the read was used as the left
hand expression of the ternary "? :" operator

 - as a result, the compiler decided that there's no dependency

 - the compiler didn't think about the dependency that comes from the
result of the load *also* being used as the middle part of the ternary
expression, because it had optimized it away, despite the standard not
talking about that at all.

 - so the compiler never saw the dependency that the standard talks about

BECAUSE THE STANDARD LANGUAGE IS PURE AND UTTER SHIT.

My suggested language never had any of these problems, because *my*
suggested semantics are clear, logical, and don't have these kinds of
idiotic pit-falls.

Solution: Fix the f*cking C standard. No excuses, no explanations.
Just get it fixed.

                      Linus
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