[llvmlinux] Fwd: Re: [GIT PULL] locking fix

Behan Webster behanw at converseincode.com
Mon Oct 28 18:52:02 UTC 2013


And another win. :)

Behan

-------- Original Message --------
Subject: 	Re: [GIT PULL] locking fix
Date: 	Sun, 27 Oct 2013 10:28:05 -0700
From: 	Linus Torvalds <torvalds at linux-foundation.org>
To: 	Ingo Molnar <mingo at kernel.org>, Maarten Lankhorst
<maarten.lankhorst at canonical.com>
CC: 	Linux Kernel Mailing List <linux-kernel at vger.kernel.org>, Peter
Zijlstra <a.p.zijlstra at chello.nl>, Thomas Gleixner <tglx at linutronix.de>,
Andrew Morton <akpm at linux-foundation.org>



On Sat, Oct 26, 2013 at 5:19 AM, Ingo Molnar <mingo at kernel.org> wrote:
>
> This tree fixes a boot crash in CONFIG_DEBUG_MUTEXES=y kernels, on
> kernels built with GCC 3.x. (There are still such distros.)

Btw, it's really not just gcc 3.x. That code was (a) incomprehensible,
(b) wrong and (c) caused problems for LLVM too.

It was wrong because "__builtin_constant_p(ww_ctx == NULL)" simply
makes no sense.

Why?

That expression is largely equivalent to
"__builtin_constant_p(ww_ctx)" (because iff ww_ctx is constant, then
the comparison to NULL is constant), which is actually much easier to
read, while carrying a totally different semantic meaning. Making
things worse, the comparison to NULL *may* be marked constant under
some very random situations (ie the compiler could turn a "taking an
address of a variable is never NULL" kind of knowledge and combining
it with other knowledge, and turn a complicated "ctx" expression into
a "I know this cannot be NULL" thing, and thus the "== NULL" is a
constant, even though ctx itself is some dynamic calculation).

Whoever wrote the original should be shot. And this commit shouldn't
have been marked as being somehow about gcc-version dependence, but
about removing completely crap code.

                    Linus
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