linux-kernel - Re: Random panic in load

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Message-ID: <CA+55aFyM1wqTLK0m4GQJ28f=z0iZE87=xk+1z-vm+PH1_d8nKw@mail.gmail.com>
Date:	Wed, 23 Jul 2014 12:20:55 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Michel Dänzer <michel@...nzer.net>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Wed, Jul 23, 2014 at 12:02 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Here goes..

Oh. So this doesn't have CPUMASK_OFFSTACK set at all, so the pointer
has never been loaded from memory in the first place. The calculation
has been (for me) something like

        movq    $load_balance_mask, %rax
        add %gs:this_cpu_off, %rax

and then gcc is being stupid and saving it to the frame and reloading
it for no good reason (at least for me it *also* saved the value in
%rbx in order to save it into "env.cpus", and the stack spill seems to
be just moronic).

In Michel's oops, %rbx doesn't contain the pointer any more, though,
so he clearly does have a different compiler. His frame offsets are
rather different too ("-136(%rbp)" vs "-168(%rbp)") so looking at
whether possibly some stack frame got overwritten is clearly very
compiler-specific.

Michel, mind doing

    make kernel/sched/fair.s

and sending us the resulting file?

              Linus
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