Re: [ELISA Safety Architecture WG] [PATCH v2 0/2] Introduce the pkill_on_warn parameter

[Lukas Bulwahn]

On Tue, Nov 16, 2021 at 9:41 AM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> On Tue 2021-11-16 10:52:39, Alexander Popov wrote:
> > On 15.11.2021 18:51, Gabriele Paoloni wrote:
> > > On 15/11/2021 14:59, Lukas Bulwahn wrote:
> > > > On Sat, Nov 13, 2021 at 7:14 PM Alexander Popov <alex.popov@xxxxxxxxx> wrote:
> > > > > On 13.11.2021 00:26, Linus Torvalds wrote:
> > > > > > On Fri, Nov 12, 2021 at 10:52 AM Alexander Popov <alex.popov@xxxxxxxxx> wrote:
> > > > > Killing the process that hit a kernel warning complies with the Fail-Fast
> > > > > principle [1]. pkill_on_warn sysctl allows the kernel to stop the process when
> > > > > the **first signs** of wrong behavior are detected.
> > > > >
> > > > In summary, I am not supporting pkill_on_warn. I would support the
> > > > other points I mentioned above, i.e., a good enforced policy for use
> > > > of warn() and any investigation to understand the complexity of
> > > > panic() and reducing its complexity if triggered by such an
> > > > investigation.
> > >
> > > Hi Alex
> > >
> > > I also agree with the summary that Lukas gave here. From my experience
> > > the safety system are always guarded by an external flow monitor (e.g. a
> > > watchdog) that triggers in case the safety relevant workloads slows down
> > > or block (for any reason); given this condition of use, a system that
> > > goes into the panic state is always safe, since the watchdog would
> > > trigger and drive the system automatically into safe state.
> > > So I also don't see a clear advantage of having pkill_on_warn();
> > > actually on the flip side it seems to me that such feature could
> > > introduce more risk, as it kills only the threads of the process that
> > > caused the kernel warning whereas the other processes are trusted to
> > > run on a weaker Kernel (does killing the threads of the process that
> > > caused the kernel warning always fix the Kernel condition that lead to
> > > the warning?)
> >
> > Lukas, Gabriele, Robert,
> > Thanks for showing this from the safety point of view.
> >
> > The part about believing in panic() functionality is amazing :)
>
> Nothing is 100% reliable.
>
> With printk() maintainer hat on, the current panic() implementation
> is less reliable because it tries hard to provide some debugging
> information, for example, error message, backtrace, registry,
> flush pending messages on console, crashdump.
>
> See panic() implementation, the reboot is done by emergency_restart().
> The rest is about duping the information.
>
> Well, the information is important. Otherwise, it is really hard to
> fix the problem.
>
> From my experience, especially the access to consoles is not fully
> safe. The reliability might improve a lot when a lockless console
> is used. I guess that using non-volatile memory for the log buffer
> might be even more reliable.
>
> I am not familiar with the code under emergency_restart(). I am not
> sure how reliable it is.
>
> > Yes, safety critical systems depend on the robust ability to restart.
>
> If I wanted to implement a super-reliable panic() I would
> use some external device that would cause power-reset when
> the watched device is not responding.
>

Petr, that is basically the common system design taken.

The whole challenge then remains to show that:

Once panic() was invoked, the watched device does not signal being
alive unintentionally, while the panic() is stuck in its shutdown
routines. That requires having a panic() or other shutdown routine
that still reliably can do something that the kernel routine that
makes the watched device signal does not signal anymore.


Lukas

> Best Regards,
> Petr
>
>
> PS: I do not believe much into the pkill approach as well.
>
>     It is similar to OOM killer. And I always had to restart the
>     system when it was triggered.
>
>     Also kernel is not prepared for the situation that an external
>     code kills a kthread. And kthreads are used by many subsystems
>     to handle work that has to be done asynchronously and/or in
>     process context. And I guess that kthreads are non-trivial
>     source of WARN().