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SubjectRe: [PATCH] PM-runtime: fix deadlock with ktime
On Wed, 30 Jan 2019 at 10:39, Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Wed, Jan 30, 2019 at 10:14 AM Vincent Guittot
> <vincent.guittot@linaro.org> wrote:
> >
> > 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
>
> So that should be explained in the changelog of the patch. In detail,
> if poss, please.

Ok, I'm going to update the commit message

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