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

[Rafael J. Wysocki]

On Wed, Jan 30, 2019 at 10:14 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> Hi Geert,
>
> On Wed, 30 Jan 2019 at 09:21, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> >
> > 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?
>
> 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.