linux-kernel - Re: Random panic in load

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Message-ID: <20140726193557.GA21842@x4>
Date:	Sat, 26 Jul 2014 21:35:57 +0200
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Michel Dänzer <michel@...nzer.net>,
	Jakub Jelinek <jakub@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Debian GCC Maintainers <debian-gcc@...ts.debian.org>,
	Debian Kernel Team <debian-kernel@...ts.debian.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On 2014.07.26 at 11:39 -0700, Linus Torvalds wrote:
> On Sat, Jul 26, 2014 at 11:28 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > That's a bit worrisome. I haven't actually checked if the code
> > generation differs in significant ways yet..
> 
> Nope. Just three instructions that got re-ordered from ABC to CAB in a
> way that makes no difference. But just the knowledge that "-g" affects
> code generation is nasty.  And with "allmodconfig" my build fails
> almost immediately (failures on at least arch/x86/kernel/process_64.c,
> kernel/exit.c and mm/vmalloc.c in that case. I was too lazy to check
> what the differences were).
> 
> Does anybody have current gcc build and can verify that current gcc
> tip passes the kernel compile with that
> 
>     export GCC_COMPARE_DEBUG=1
> 
> thing?

The fs/ext4/inode.c issue is a variant of the gcc bug and isn't fixed
yet. I will open a bug report. The kernel/exit.c issue is already fixed,
see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 for a reduced
testcase. 

But fortunately the workaround for the new inode.c bug is the same as
for the original bug: -fno-var-tracking-assignments. 

It would make sense to enabled it unconditionally for all debug
configurations for now.

-- 
Markus
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