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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 29 Apr 2016 17:09:56 -0700
Message-ID: <CALCETrXC5mVPTKjW7gZG_GGbADbeUPO7X-9ARTOaCVo1icYJkw@mail.gmail.com>
Subject: Re: [RFC PATCH v2 05/18] sched: add task flag for preempt IRQ tracking
To: Jiri Kosina <jikos@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>, Josh Poimboeuf <jpoimboe@redhat.com>,
        X86 ML <x86@kernel.org>, Heiko Carstens <heiko.carstens@de.ibm.com>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
        live-patching@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
        Chris J Arges <chris.j.arges@canonical.com>,
        linuxppc-dev@lists.ozlabs.org, Jessica Yu <jeyu@redhat.com>,
        Petr Mladek <pmladek@suse.com>, Jiri Slaby <jslaby@suse.cz>,
        Vojtech Pavlik <vojtech@suse.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Miroslav Benes <mbenes@suse.cz>, Peter Zijlstra <peterz@infradead.org>
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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