linux-kernel - Re: [PATCH 02/17] mm: mmu

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Message-ID: <AANLkTimvHdGZptwmmw73C2jsy=HqgreEAxNurT1Hxbv=@mail.gmail.com>
Date:	Fri, 1 Apr 2011 09:13:51 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Avi Kivity <avi@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Nick Piggin <npiggin@...nel.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>, Tony Luck <tony.luck@...el.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 02/17] mm: mmu_gather rework

On Fri, Apr 1, 2011 at 5:07 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> No, although I do try to avoid it in structures because I'm ever unsure
> of the storage type used. But yes, good suggestion, thanks!

I have to admit to not being a huge fan of "bool". You never know what
it actually is in C, and it's a possible source of major confusion.

Some environments will make it "int", others "char", and others - like
the kernel - will make it a C99/C++-like "true boolean" (C99 _Bool).

What's the difference? Integer assignment makes a hell of a difference. Do this:

  long long expression = ...
  ...
  bool val = expression;

and depending on implementation it will either just truncate the value
to a random number of bits, or actually do a compare with zero.

And while we use the C99 _Bool type, and thus get those true boolean
semantics (ie not just be a truncated integer type), I have to say
that it's still a dangerous thing to do in C because you generally
cannot rely on it. There's _tons_ of software that just typedefs int
or char to bool.

So even outside of structures, I'm not necessarily convinced "bool" is
always such a good thing. But I'm not going to stop people from using
it (inside the kernel it should be safe), I just want to raise a
warning and ask people to not use it mindlessly. And avoid the casts -
even if they are safe in the kernel.

                      Linus
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