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:	Thu, 20 Nov 2014 00:32:40 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andy Lutomirski <luto@...capital.net>,
	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, 19 Nov 2014, Linus Torvalds wrote:

> 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.

preempt_count is a per cpu variable at least on x86 since:

  commit c2daa3bed53a 'sched, x86: Provide a per-cpu preempt_count
  implementation'
    
> > 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.

do_nmi()
  nmi_enter()
    preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET); 

> (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)

Certainly something you should not smoke again :)

Thanks,

	tglx
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