linux-kernel - Re: [GIT PULL] locking fix

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Message-ID: <526D6FF9.9020003@canonical.com>
Date:	Sun, 27 Oct 2013 20:56:41 +0100
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] locking fix

op 27-10-13 20:51, Linus Torvalds schreef:
> On Sun, Oct 27, 2013 at 12:37 PM, Maarten Lankhorst
> <maarten.lankhorst@...onical.com> wrote:
>> I would love for a compiler to become that smart though, but I do not think it's likely.
> Dammit, even if that is true, then write the conditional *correctly*.
>
> As mentioned, the conditional
>
>     __builtin_constant_p(ww_ctx) && ww_ctx == NULL
>
> is actually sensible, in a way the original one was *not*. It actually
> tests what you apparently intended to test, and is more readable to
> humans to boot.
Yeah that mail arrived after I sent mine, I agree that this would have been more sensible.
> And no, it still isn't actually guaranteed to do what you want it to
> do. Historically, in gcc, __builtin_constant_p() really only ever
> worked in macros, because by the time you use it in inline functions,
> a constant NULL in the caller will have been turned into a argument
> variable in the inline function, and __builtin_constant_p() would be
> done before that was optimized away. Over the years, gcc has pushed
> some of the builtin evaluation deeper down, and these days it actually
> works within inline functions, but my point that
> __builtin_constant_p() is about a certain level of compiler
> optimization is very much true: you're actually testing for a compiler
> optimization detail.
>
> I know the LLVM people had similar issues with this comparison, so
> these days it's not even just about gcc versions. We may never have
> cared very much about icc, but llvm is actually an interesting target
> compiler.
>
And this is why ww_ctx == NULL is now passed as an inline argument. :)

~Maarten
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