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

[Ulf Hansson]

On Wed, 30 Jan 2019 at 12:16, 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

Hmm, isn't this already a problem in genpd then?

In genpd's ->runtime_resume() callback (genpd_runtime_resume()) we
call ktime_get() to measure the latency of running the callback.

Or, perhaps irq_safe_dev_in_no_sleep_domain() returns true for the
genpd + device in question, which means the ktime thingy is skipped.

Geert?


>                 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>

Reviewed-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx>

Kind regards
Uffe

> ---
>
> - 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);
> +       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
>