linux-kernel - Re: [PATCH v3 3/3] sched, x86: Check that we're on the right stack in schedule and __might

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Date:	Wed, 19 Nov 2014 15:23:16 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andi Kleen <andi@...stfloor.org>, Borislav Petkov <bp@...en8.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v3 3/3] sched, x86: Check that we're on the right stack in
 schedule and __might_sleep

On Wed, Nov 19, 2014 at 3:04 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> Does that include nmi?  I'm a bit afraid of touching that code.

NMI is kind of special, since it's really not supposed to touch
'current' or anything like that, and that's how we do preempt-count
(and that's where irq-count is) right now.

I would prefer to have preempt_count be a percpu variable rather than
a per-thread one, but there are historical reasons for that horror. Oh
well.

> It's certainly easy enough to bump irq_count in the paranoid entries.

It definitely shouldn't be done from the assembly code. Usually it's
"irq_enter/exit()" that does it, but for NMI you'd presumably just do
it explicitly from do_nmi() or similar. But NMI really is nastier than
other cases, see above.

(That said, I thought we did it anyway, but now that I look I can't
find it. So I was probably just smoking something)

             Linus
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