linux-kernel - Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of load

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Date:	Wed, 6 Aug 2014 09:34:56 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Jakub Jelinek <jakub@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Michel Dänzer <michel@...nzer.net>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Josh Stone <jistone@...hat.com>
Subject: Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of load_balance()
 in scheduler

On Wed, Aug 6, 2014 at 1:14 AM, Jakub Jelinek <jakub@...hat.com> wrote:
> On Tue, Aug 05, 2014 at 03:36:39PM -0700, Linus Torvalds wrote:
d>> I don't understand how you guys can be so cavalier about a compiler
>> bug that has already resulted in actual real problems. You bring up
>
> I have no problem with a -fno-var-tracking-assignments workaround for
> compilers that have the PR61801 wrong-code bug.  What I have problem with
> is with disabling it even for compilers that have that bug fixed.
> That is in essence disabling a useful feature just because it could have
> other bugs.  If my memory serves me well, PR61801 is the only wrong-code
> I remember caused by -fvar-tracking-assignments during the 5 years since
> it has been introduced into gcc.  Sure, there have been several
> -fcompare-debug bugs, where we generated slightly different code between
> -g and -g0, and as you mentioned we have one still pending (Vladimir is
> working on it right now), but that is mainly relevant to the case where

I think gcc guys are taking a wrong lesson out of this. kernel doesn't
care too much whether gcc produces the same binary with -g and -g0.
kernel developers also don't care about amount debug info for variables,
but they care about hard to find compiler bugs. In this case sched2
mishap around debug_insn was a symptom. The root cause is lack
of attention to -mno-red-zone. Kernel is not another user space program
where data/control flow analysis is all compiler need to make things right.
-mno-red-zone lesson exposes lack of 'interrupt' concept in compiler.
I think there has to be some infra put in place to make sure that it's not
just a scheduling barrier. Otherwise next bug will pop much sooner
than 5 years and it will not be related to debug info at all.
In this sense Steven's perl script to detect red-zone violations did more
to re-enable var-tracking than -fcompare-debug fixes.
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