linux-kernel - Re: Random panic in load

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Message-ID: <CA+55aFzSU5u8MiGqRZ6BVojQPnxbd1hxmFY_BA=UuCAygxR8WQ@mail.gmail.com>
Date:	Wed, 23 Jul 2014 09:54:23 -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 8:55 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> I haven't seen the full oops, can you forward the screenshot? The
>> exact register state might give some clues.
>
> Sure, here goes.

So the length is fine, and the disassembly shows that it is fixed (16
32-bit words - why the heck does it use "movsl" rather than "movsq",
whatever).

The problem is %rdi, which has the value ffff10043c803e8c, which isn't
canonical. Which is why it GP-faults.

That value is loaded from the stack:

        mov    -0x88(%rbp),%rdi

so apparently the original "__get_cpu_var(load_balance_mask)" is
already corrupted, or something has corrupted it on the stack since
loading (but that looks unlikely).

And I wonder if I have a clue. Look, load_balance_mask is a
"cpumask_var_t", but I don't see a "alloc_cpumask_var()" for it.
That's broken with CONFIG_CPUMASK_OFFSTACK.

I think you actually want "load_balance_mask" to be a "struct cpumask *", no?

Alternatively, keep it a "cpumask_var_t", but then you need to use
__get_cpu_pointer() to get the address of it, and use
"alloc_cpumask_var()" to allocate area for the OFFSTACK case.

TOTALLY UNTESTED AND PROBABLY PURE CRAP PATCH ATTACHED.

WARNING! WARNING! WARNING! This is just looking at the code, not
really knowing it, and saying "that looks really really wrong". Maybe
I'm full of shit.

                Linus

View attachment "patch.diff" of type "text/plain" (1293 bytes)