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SubjectRe: [PATCH] PM-runtime: fix deadlock with ktime
Hi Geert,

On Wed, 30 Jan 2019 at 09:21, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Vincent,
>
> On Wed, Jan 30, 2019 at 9:16 AM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> > A deadlock has been seen when swicthing clocksources which use PM runtime.
> > The call path is:
> > change_clocksource
> > ...
> > write_seqcount_begin
> > ...
> > timekeeping_update
> > ...
> > sh_cmt_clocksource_enable
> > ...
> > rpm_resume
> > pm_runtime_mark_last_busy
> > ktime_get
> > do
> > read_seqcount_begin
> > while read_seqcount_retry
> > ....
> > write_seqcount_end
> >
> > Although we should be safe because we haven't yet changed the clocksource
> > at that time, we can't because of seqcount protection.
> >
> > Use ktime_get_mono_fast_ns instead which is lock safe for such case
> >
> > Fixes: 8234f6734c5d ("PM-runtime: Switch autosuspend over to using hrtimers")
> > Reported-by: Biju Das <biju.das@bp.renesas.com>
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
>
> Thanks for your patch!
>
> /**
> * ktime_get_mono_fast_ns - Fast NMI safe access to clock monotonic
> *
> * This timestamp is not guaranteed to be monotonic across an update.
> * The timestamp is calculated by:
> *
> * now = base_mono + clock_delta * slope
> *
> * So if the update lowers the slope, readers who are forced to the
> * not yet updated second array are still using the old steeper slope.
> *
> * tmono
> * ^
> * | o n
> * | o n
> * | u
> * | o
> * |o
> * |12345678---> reader order
> *
> * o = old slope
> * u = update
> * n = new slope
> *
> * So reader 6 will observe time going backwards versus reader 5.
> *
> * While other CPUs are likely to be able observe that, the only way
> * for a CPU local observation is when an NMI hits in the middle of
> * the update. Timestamps taken from that NMI context might be ahead
> * of the following timestamps. Callers need to be aware of that and
> * deal with it.
> */
>
> As this function is not guaranteed to be monotonic, have you checked how
> the Runtime PM code behaves if time goes backwards? Does it just make
> a suboptimal decision or does it crash?

As a worst case this will generate a suboptimal decision around the update

Regards,
Vincent

>
> Thanks!
>
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
> -- Linus Torvalds

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