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Date: Wed, 30 Jan 2019 20:39:25 +0100
From: Ladislav Michl <ladis@...ux-mips.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Linux PM <linux-pm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Ulf Hansson <ulf.hansson@...aro.org>,
Biju Das <biju.das@...renesas.com>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: Re: [PATCH v2 ] PM-runtime: fix deadlock with ktime
On Wed, Jan 30, 2019 at 02:18:49PM +0100, Vincent Guittot wrote:
> On Wed, 30 Jan 2019 at 14:06, Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Wed, Jan 30, 2019 at 12:16 PM Vincent Guittot
> > <vincent.guittot@...aro.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
> > >
> > > 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@...renesas.com>
> > > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >
> > 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.
>
> Thanks for adding Ladislav in this thread.
> I'm sorry I forgot to add him in the loop.
>
> >
> > Can you or Ladislav please rebase that patch on top of this one and repost?
>
> Ladislav,
>
> Let me know if you prefer to rebase and repost your patch of if you
> want me to do.
I'll rebase it on top of Rafael's bleeding-edge branch.
Best regards,
ladis
> Regards,
> Vincent
>
> >
> > > ---
> > >
> > > - 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
> > >
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