linux-kernel - Re: frequent lockups in 3.18rc4

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Message-ID: <20141118020959.GA2091@redhat.com>
Date:	Mon, 17 Nov 2014 21:09:59 -0500
From:	Dave Jones <davej@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: frequent lockups in 3.18rc4

On Mon, Nov 17, 2014 at 11:59:34AM -0800, Linus Torvalds wrote:
 > On Mon, Nov 17, 2014 at 9:03 AM, Dave Jones <davej@...hat.com> wrote:
 > >
 > > It could a completely different cause for lockup, but seeing this now
 > > has me wondering if perhaps it's something unrelated to the kernel.
 > > I have recollection of running late .17rc's for days without incident,
 > > and I'm pretty sure .17 was ok too.  But a few weeks ago I did upgrade
 > > that test box to the Fedora 21 beta.  Which means I have a new gcc.
 > > I'm not sure I really trust 4.9.1 yet, so maybe I'll see if I can
 > > get 4.8 back on there and see if that's any better.
 > 
 > I'm not sure if I should be relieved or horrified.
 > 
 > Horrified, I think.
 > 
 > It really would be a wonderful thing to have some kind of "compiler
 > bisection" with mixed object files to see exactly which file it
 > miscompiles (and by "miscompiles" it might just be a kernel bug where
 > we are missing a barrier or something, and older gcc's just happened
 > to not show it - so it could still easily be a kernel problem).

After wasting countless hours rolling back to Fedora 20 and gcc 4.8.1,
I saw the exact same trace on 3.17, so now I don't know what to think.

So it's great that it's not a regression vs .17, but otoh, who knows
how far back this goes. This looks like a nightmarish bisect case, and
I've no idea why it's now happening so often.

I'll give Don's softlockup_all_cpu_backtrace=1 idea a try on 3.18rc5
and see if that shines any more light on this.

Deeply puzzling.

	Dave

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