Re: [PATCH] PM-runtime: fix deadlock with ktime

[Geert Uytterhoeven]

Hi Vincent,

On Wed, Jan 30, 2019 at 9:16 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

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?

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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