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Re: [RFC][PATCH 0/5] arch: atomic rework
- From: Linus Torvalds <torvalds at linux-foundation dot org>
- To: Paul McKenney <paulmck at linux dot vnet dot ibm dot com>
- Cc: Torvald Riegel <triegel at redhat dot com>, Will Deacon <will dot deacon at arm dot com>, Peter Zijlstra <peterz at infradead dot org>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, David Howells <dhowells at redhat dot com>, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>, "akpm at linux-foundation dot org" <akpm at linux-foundation dot org>, "mingo at kernel dot org" <mingo at kernel dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Thu, 20 Feb 2014 10:32:51 -0800
- Subject: Re: [RFC][PATCH 0/5] arch: atomic rework
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On Thu, Feb 20, 2014 at 10:11 AM, Paul E. McKenney
<paulmck@linux.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