Re: frequent lockups in 3.18rc4

From: Dave Jones
Date: Mon Nov 17 2014 - 21:10:22 EST

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@xxxxxxxxxx> 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.


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