Messages in this thread | | | Date | Tue, 5 Jun 2018 09:17:52 +0200 (CEST) | From | Miroslav Benes <> | Subject | Re: [PATCH 1/2] livepatch: Send a fake signal periodically |
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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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