Re: [PATCH v2 ] PM-runtime: fix deadlock with ktime

[Ladislav Michl]

On Wed, Jan 30, 2019 at 02:06:07PM +0100, Rafael J. Wysocki wrote:
> On Wed, Jan 30, 2019 at 12:16 PM 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
> >
> > With ktime_get_mono_fast_ns, the timestamp is not guaranteed to be
> > monotonic across an update and as a result can goes backward. According to
> > update_fast_timekeeper() description: "In the worst case, this can
> > result is a slightly wrong timestamp (a few nanoseconds)". For
> > PM runtime autosuspend, this means only that the suspend decision can
> > be slightly sub optimal.
> >
> > 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>
> 
> I've queued this one up as a fix for 5.0, but unfortunately it clashes
> with the patch from Ladislav Michl at
> https://patchwork.kernel.org/patch/10755477/ which has been dropped
> now.
> 
> Can you or Ladislav please rebase that patch on top of this one and repost?
> 
> > ---
> >
> > - v2: Updated commit message to explain the impact of using
> >   ktime_get_mono_fast_ns()
> >
> >  drivers/base/power/runtime.c | 10 +++++-----
> >  include/linux/pm_runtime.h   |  2 +-
> >  2 files changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
> > index 457be03..708a13f 100644
> > --- a/drivers/base/power/runtime.c
> > +++ b/drivers/base/power/runtime.c
> > @@ -130,7 +130,7 @@ u64 pm_runtime_autosuspend_expiration(struct device *dev)
> >  {
> >         int autosuspend_delay;
> >         u64 last_busy, expires = 0;
> > -       u64 now = ktime_to_ns(ktime_get());
> > +       u64 now = ktime_get_mono_fast_ns();
> >
> >         if (!dev->power.use_autosuspend)
> >                 goto out;
> > @@ -909,7 +909,7 @@ static enum hrtimer_restart  pm_suspend_timer_fn(struct hrtimer *timer)
> >          * If 'expires' is after the current time, we've been called
> >          * too early.
> >          */
> > -       if (expires > 0 && expires < ktime_to_ns(ktime_get())) {
> > +       if (expires > 0 && expires < ktime_get_mono_fast_ns()) {
> >                 dev->power.timer_expires = 0;
> >                 rpm_suspend(dev, dev->power.timer_autosuspends ?
> >                     (RPM_ASYNC | RPM_AUTO) : RPM_ASYNC);
> > @@ -928,7 +928,7 @@ static enum hrtimer_restart  pm_suspend_timer_fn(struct hrtimer *timer)
> >  int pm_schedule_suspend(struct device *dev, unsigned int delay)
> >  {
> >         unsigned long flags;
> > -       ktime_t expires;
> > +       u64 expires;
> >         int retval;
> >
> >         spin_lock_irqsave(&dev->power.lock, flags);
> > @@ -945,8 +945,8 @@ int pm_schedule_suspend(struct device *dev, unsigned int delay)
> >         /* Other scheduled or pending requests need to be canceled. */
> >         pm_runtime_cancel_pending(dev);
> >
> > -       expires = ktime_add(ktime_get(), ms_to_ktime(delay));
> > -       dev->power.timer_expires = ktime_to_ns(expires);
> > +       expires = ktime_get_mono_fast_ns() + (u64)delay * NSEC_PER_MSEC);

Just FYI, this one does not compile                                        ^
I'm going to send updated optimization patch anyway as fixing this one will not
break it :)

> > +       dev->power.timer_expires = expires;
> >         dev->power.timer_autosuspends = 0;
> >         hrtimer_start(&dev->power.suspend_timer, expires, HRTIMER_MODE_ABS);
> >
> > diff --git a/include/linux/pm_runtime.h b/include/linux/pm_runtime.h
> > index 54af4ee..fed5be7 100644
> > --- a/include/linux/pm_runtime.h
> > +++ b/include/linux/pm_runtime.h
> > @@ -105,7 +105,7 @@ static inline bool pm_runtime_callbacks_present(struct device *dev)
> >
> >  static inline void pm_runtime_mark_last_busy(struct device *dev)
> >  {
> > -       WRITE_ONCE(dev->power.last_busy, ktime_to_ns(ktime_get()));
> > +       WRITE_ONCE(dev->power.last_busy, ktime_get_mono_fast_ns());
> >  }
> >
> >  static inline bool pm_runtime_is_irq_safe(struct device *dev)
> > --
> > 2.7.4
> >