linux-kernel - Re: [PATCH] RTC: Avoid races between RTC alarm wakeup and suspend.

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Date:	Tue, 31 Jul 2012 12:51:04 +1000
From:	NeilBrown <neilb@...e.de>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Paul Fox <pgf@...top.org>
Subject: Re: [PATCH] RTC: Avoid races between RTC alarm wakeup and suspend.

On Mon, 30 Jul 2012 23:01:49 +0200 "Rafael J. Wysocki" <rjw@...k.pl> wrote:

> On Monday, July 30, 2012, NeilBrown wrote:
> > 
> > If an RTC alarm fires just as suspend is happening, it is possible for
> > suspend to complete and the alarm to be missed.
> > 
> > To avoid the race, we must register the event with the PM core.
> > 
> > As the event is made visible to userspace through a thread which is
> > only scheduled by the interrupt, we need a pm_stay_awake/pm_relax
> > pair preventing suspend from the interrupt until the thread completes
> > its work.
> > 
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > 
> > --
> > This makes the pm_wakeup_event() call in cmos_interrupt unnecessary as it
> > provides suspend protection for all RTCs that use rtc_update_irq.
> 
> Care to remove the call in cmos_interrupt(), then?
> 
> > I think the pm_stay_awake//pm_relax is needed - just pm_wakup_event() is 
> > theoretically not sufficient.
> > 
> > This is because there is no guarantee (that I know of) that the workqueue
> > thread will actually get scheduled before 'suspend' takes over.
> 
> I think you are right.

Thanks.
Here is the revised patch.

NeilBrown


==========
Subject: [PATCH] RTC: Avoid races between RTC alarm wakeup and suspend.

If an RTC alarm fires just as suspend is happening, it is possible for
suspend to complete and the alarm to be missed.

To avoid the race, we must register the event with the PM core.

As the event is made visible to userspace through a thread which is
only scheduled by the interrupt, we need a pm_stay_awake/pm_relax
pair preventing suspend from the interrupt until the thread completes
its work.

This makes the pm_wakeup_event() call in cmos_interrupt unnecessary as
it provides suspend protection for all RTCs that use rtc_update_irq.

Cc: Paul Fox <pgf@...top.org>
Signed-off-by: NeilBrown <neilb@...e.de>

diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index eb415bd..9592b93 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -582,6 +582,7 @@ enum hrtimer_restart rtc_pie_update_irq(struct hrtimer *timer)
 void rtc_update_irq(struct rtc_device *rtc,
 		unsigned long num, unsigned long events)
 {
+	pm_stay_awake(rtc->dev.parent);
 	schedule_work(&rtc->irqwork);
 }
 EXPORT_SYMBOL_GPL(rtc_update_irq);
@@ -844,6 +845,7 @@ void rtc_timer_do_work(struct work_struct *work)
 
 	mutex_lock(&rtc->ops_lock);
 again:
+	pm_relax(rtc->dev.parent);
 	__rtc_read_time(rtc, &tm);
 	now = rtc_tm_to_ktime(tm);
 	while ((next = timerqueue_getnext(&rtc->timerqueue))) {
diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index 132333d..4267789 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -568,7 +568,6 @@ static irqreturn_t cmos_interrupt(int irq, void *p)
 		hpet_mask_rtc_irq_bit(RTC_AIE);
 
 		CMOS_READ(RTC_INTR_FLAGS);
-		pm_wakeup_event(cmos_rtc.dev, 0);
 	}
 	spin_unlock(&rtc_lock);
 

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