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SubjectRe: [PATCH 1/2] livepatch: Send a fake signal periodically
On Mon, 4 Jun 2018, Josh Poimboeuf wrote:

> On Mon, Jun 04, 2018 at 04:16:35PM +0200, Miroslav Benes wrote:
> > An administrator may send a fake signal to all remaining blocking tasks
> > of a running transition by writing to
> > /sys/kernel/livepatch/<patch>/signal attribute. Let's do it
> > automatically after 10 seconds. The timeout is chosen deliberately. It
> > gives the tasks enough time to transition themselves.
> >
> > Theoretically, sending it once should be more than enough. Better be safe
> > than sorry, so send it periodically.
>
> This is the part I don't understand. Why do it periodically?

I met (rare!) cases when doing it once was not enough due to a race and
the signal was missed. However involved testcases were really artificial.

> Instead, might it make sense to just send the signals once, and if that
> doesn't work, reverse the transition? Then we could make patching a
> synchronous operation. But then, it might be remotely possible that the
> reverse operation also stalls (e.g., on a kthread). So, maybe it's best
> to just leave all these controls in the hands of the user.

And there is 'force' option...

So given all this, I'd call klp_send_signals() once and then leave it up
to the user. Would that work for you?

> All that said, a few code review comments:
>
> - AFAICT, it does an 8 second delay instead of a 10 second delay,
> because

Not that it matters, because it is still wrong, but it is a 9 second
delay (or I miscounted again).

> a) try_complete_transition() is first called before there's any delay;
>
> b) the preincrement operator used on signals_cnt.
>
> - I think 15 seconds might be a better default. I've seen longer
> patching delays on a system with 100+ CPUs.

Ok, why not.

> - If a kthread or idle task is sleeping on a patched function, the
> pr_notice("signaling remaining tasks\n") will be repeated continously.

True.

> - It might be cleaner to do it from the delayed work function
> (klp_transition_work_fn).

I considered it but then decided to do it in klp_try_complete_transition()
under 'if (!complete)'. It belongs right there in my opinion.

Thanks,
Miroslav

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