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

[Vincent Guittot]

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>
---

- 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