From: Maarten Lankhorst <maarten.lankhorst@canonical.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Thomas Gleixner <tglx@linutronix.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [GIT PULL] locking fix
Date: Sun, 27 Oct 2013 20:00:40 +0100 [thread overview]
Message-ID: <526D62D8.8050001@canonical.com> (raw)
In-Reply-To: <CA+55aFwLwZ2SoqLdD9itnXrFJyS9szC+g4J01pLxHFgKYCvrog@mail.gmail.com>
op 27-10-13 18:28, Linus Torvalds schreef:
> On Sat, Oct 26, 2013 at 5:19 AM, Ingo Molnar <mingo@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.
>
Unfortunately gcc disagreed there, which was another compiler bug.
__builtin_constant_p(ww_ctx) was NOT equal to __builtin_constant_p(ww_ctx == NULL), iirc.
__builtin_constant_p(ww_ctx == NULL) is equal to __builtin_constant_p(ww_ctx != NULL), but
the former is more readable, since it shows we expect ww_ctx to be null.
But yeah I guess it was too broken in gcc after all, so that's why it had to be killed altogether.
next prev parent reply other threads:[~2013-10-27 19:00 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-26 12:19 [GIT PULL] locking fix Ingo Molnar
2013-10-27 17:28 ` Linus Torvalds
2013-10-27 19:00 ` Maarten Lankhorst [this message]
2013-10-27 19:23 ` Linus Torvalds
2013-10-27 19:35 ` Linus Torvalds
2013-10-27 19:37 ` Maarten Lankhorst
2013-10-27 19:51 ` Linus Torvalds
2013-10-27 19:56 ` Maarten Lankhorst
2013-10-27 19:59 ` Linus Torvalds
2013-10-28 8:47 ` Ingo Molnar
-- strict thread matches above, loose matches on Subject: below --
2024-04-14 8:01 Ingo Molnar
2024-04-14 18:48 ` pr-tracker-bot
2023-11-26 9:39 Ingo Molnar
2023-11-26 17:16 ` pr-tracker-bot
2023-02-11 8:54 Ingo Molnar
2023-02-11 19:24 ` pr-tracker-bot
2021-03-28 10:28 Ingo Molnar
2021-03-28 19:22 ` pr-tracker-bot
2019-07-14 11:36 Ingo Molnar
2019-07-14 18:45 ` pr-tracker-bot
2019-05-16 16:01 Ingo Molnar
2019-05-16 17:57 ` Linus Torvalds
2019-05-16 18:39 ` Greg KH
2019-05-16 18:42 ` Linus Torvalds
2019-05-16 23:55 ` Sasha Levin
2019-05-17 12:16 ` Greg KH
2019-05-16 18:20 ` pr-tracker-bot
2019-04-12 11:53 Ingo Molnar
2019-04-13 4:05 ` pr-tracker-bot
2017-07-21 10:11 Ingo Molnar
2016-09-13 18:11 Ingo Molnar
2016-04-16 9:16 Ingo Molnar
2015-08-14 7:08 Ingo Molnar
2015-03-28 10:07 Ingo Molnar
2015-03-01 16:57 Ingo Molnar
2011-02-15 17:02 Ingo Molnar
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