Hi Jeffy,

A few corrections here. (Sorry, I didn't completely reread the driver
here before sending.)

On Mon, Feb 26, 2018 at 10:01:15AM -0800, Brian Norris wrote:

On Sun, Feb 25, 2018 at 04:18:02PM +0800, Jeffy Chen wrote:

quoted

We have a check in __rtc_set_alarm() to return -ETIME when the alarm
is in the past.

Since accessing a Chrome OS EC based rtc is a slow operation, we should
do that check again inside of the EC rtc driver's .set_alarm() callback.

Thanks for the patch. I'd note that this is related to the race
documented in __rtc_set_alarm() (drivers/rtc/interface.c):

        /*
         * XXX - We just checked to make sure the alarm time is not
         * in the past, but there is still a race window where if
         * the is alarm set for the next second and the second ticks
         * over right here, before we set the alarm.
         */

It feels like we should put this comment somewhere more prominent;
perhaps some kerneldoc for the .set_alarm() callback? Because I suspect
that nearly every RTC driver is susceptible to this problem.

Anyway, I think this patch is helpful, because as you note the EC
protocol is relatively slow (it's much more than just a register write),
but your patch still doesn't really cover the whole problem. Even if you
compare the current time here, time marches on between here and
EC_CMD_RTC_SET_ALARM. So you can still have the same race, where the RTC
makes another tick before we set the alarm? Just think: what if we slept
for a second right after that -ETIME check?

What happens next...depends on the implementation I suppose. It's
possible that an alarm could still immediately fire for a "past" event.
But it's also possible the alarm will get dropped [1].

In the particular case of this driver...we're actually OK because the
alarm time is programmed via an offset. So as long as we give it a
postive number, we're in the clear. We might set a longer than-expected
alarm I suppose, but that's not the end of the world...

I wonder if a better solution would be to re-check the clock right after
setting the alarm. If the alarm is already past, then we should return
-ETIME? Is there any harm in double-reporting an alarm? (If so, we could
try to add accounting information somehow...)

I also wonder if that check should be done in the generic code (perhaps
with a flag to opt-in or opt-out?), since this really seems like a
fundamental problem of the interface.

Given we actually don't need this approach for the CrOS EC code, it
definitely would need to be possible to disable such code ;) But that
still doesn't mean other RTC drivers are safe.

One more note below:

Brian

[1] And lest we think that dropping it is fine: this breaks, e.g.,
hwclock which relies on RTC_UIE_ON -> rtc_update_irq_enable(), which
sets a 1-second alarm and expects it to fire an interrupt.

quoted

Signed-off-by: Jeffy Chen <redacted>
---

 drivers/rtc/rtc-cros-ec.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/rtc/rtc-cros-ec.c b/drivers/rtc/rtc-cros-ec.c
index f0ea6899c731..ee0062e2d222 100644
--- a/drivers/rtc/rtc-cros-ec.c
+++ b/drivers/rtc/rtc-cros-ec.c

@@ -188,6 +188,10 @@ static int cros_ec_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 	if (alarm_time < 0 || alarm_time > U32_MAX)
 		return -EINVAL;
 
+	/* Don't set an alarm in the past. */
+	if ((u32)alarm_time <= current_time)
+		return -ETIME;

I don't think we want this check on the 'disabled' case. Perhaps just
keep this under the 'else' below still?

In fact, there are *no* callers of __rtc_set_alarm() with
!alrm->enabled, but still, since this driver is *trying* to account for
that, it seems wise to retain that attempt (or else remove it entirely).

Brian

quoted

+
 	if (!alrm->enabled) {
 		/*
 		 * If the alarm is being disabled, send an alarm

@@ -196,11 +200,7 @@ static int cros_ec_rtc_set_alarm(struct device *dev, struct rtc_wkalrm *alrm)
 		alarm_offset = EC_RTC_ALARM_CLEAR;
 		cros_ec_rtc->saved_alarm = (u32)alarm_time;
 	} else {
-		/* Don't set an alarm in the past. */
-		if ((u32)alarm_time < current_time)
-			alarm_offset = EC_RTC_ALARM_CLEAR;
-		else
-			alarm_offset = (u32)alarm_time - current_time;
+		alarm_offset = (u32)alarm_time - current_time;
 	}
 
 	ret = cros_ec_rtc_set(cros_ec, EC_CMD_RTC_SET_ALARM, alarm_offset);

-- 
2.11.0