Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking

Andy Lutomirski
2016-04-30T00:10:20Z

On Apr 29, 2016 3:11 PM, "Jiri Kosina" <jikos@kernel.org> wrote:
>
> On Fri, 29 Apr 2016, Andy Lutomirski wrote:
>
> > > NMI, MCE and interrupts aren't a problem because they have dedicated
> > > stacks, which are easy to detect. If the tasks' stack is on an
> > > exception stack or an irq stack, we consider it unreliable.
> >
> > Only on x86_64.
>
> Well, MCEs are more or less x86-specific as well. But otherwise good
> point, thanks Andy.
>
> So, how does stack layout generally look like in case when NMI is actually
> running on proper kernel stack? I thought it's guaranteed to contain
> pt_regs anyway in all cases. Is that not guaranteed to be the case?
>

On x86, at least, there will still be pt_regs for the NMI. For the
interrupted state, though, there might not be pt_regs, as the NMI
might have happened while still populating pt_regs. In fact, the NMI
stack could overlap task_pt_regs.

For x86_32, there's no guarantee that pt_regs contains sp due to
hardware silliness. You need to parse it more carefully, as,
!user_mode(regs), then the old sp is just above pt_regs.

--Andy