linux-kernel - Re: [Xen-devel] Re: 2.6.39 crashes BUG: unable to handle kernel NULL pointer dereference at 000000000000042 .. cmos_checkintr+0x4d/0x55 under Xen as PV guest.

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Date:	Thu, 24 Mar 2011 13:39:34 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	tglx@...utronix.de, xen-devel@...ts.xensource.com,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] Re: 2.6.39 crashes BUG: unable to handle kernel
 NULL pointer dereference at 000000000000042 .. cmos_checkintr+0x4d/0x55
 under Xen as PV guest.

On Thu, 2011-03-24 at 12:05 -0700, John Stultz wrote:
> Or... more likely, the added rtc_set_alarm is enabling interrupts,
> allowing an irq to land before the rtc_device_register completes.
> 
> Oof.. ok. so that is a little gross.
> 
> 
> Let me see if I can't work something out here.

Konrad: Can you give this patch a shot to see if it changes the
behavior?

Basically we still initialize the alarm timer from whatever the hardware
is set to at boot time, but don't use the standard rtc_set_alarm, and
instead initialize the aie_timer and enqueue it by hand.

That should avoid having rtc_set_alarm enabling irqs possibly too early.

thanks
-john



Signed-off-by: John Stultz <john.stultz@...aro.org>

diff --git a/drivers/rtc/class.c b/drivers/rtc/class.c
index 09b4437..3901386 100644
--- a/drivers/rtc/class.c
+++ b/drivers/rtc/class.c
@@ -171,7 +171,7 @@ struct rtc_device *rtc_device_register(const char *name, struct device *dev,
 	err = __rtc_read_alarm(rtc, &alrm);
 
 	if (!err && !rtc_valid_tm(&alrm.time))
-		rtc_set_alarm(rtc, &alrm);
+		rtc_initialize_alarm(rtc, &alrm);
 
 	strlcpy(rtc->name, name, RTC_DEVICE_NAME_SIZE);
 	dev_set_name(&rtc->dev, "rtc%d", id);
diff --git a/drivers/rtc/interface.c b/drivers/rtc/interface.c
index 8ec6b06..b2fea80 100644
--- a/drivers/rtc/interface.c
+++ b/drivers/rtc/interface.c
@@ -375,6 +375,32 @@ int rtc_set_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
 }
 EXPORT_SYMBOL_GPL(rtc_set_alarm);
 
+/* Called once per device from rtc_device_register */
+int rtc_initialize_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
+{
+	int err;
+
+	err = rtc_valid_tm(&alarm->time);
+	if (err != 0)
+		return err;
+
+	err = mutex_lock_interruptible(&rtc->ops_lock);
+	if (err)
+		return err;
+
+	rtc->aie_timer.node.expires = rtc_tm_to_ktime(alarm->time);
+	rtc->aie_timer.period = ktime_set(0, 0);
+	if (alarm->enabled) {
+		rtc->aie_timer.enabled = 1;
+		timerqueue_add(&rtc->timerqueue, &rtc->aie_timer.node);
+	}
+	mutex_unlock(&rtc->ops_lock);
+	return err;
+}
+EXPORT_SYMBOL_GPL(rtc_initialize_alarm);
+
+
+
 int rtc_alarm_irq_enable(struct rtc_device *rtc, unsigned int enabled)
 {
 	int err = mutex_lock_interruptible(&rtc->ops_lock);
diff --git a/include/linux/rtc.h b/include/linux/rtc.h
index 2ca7e8a..877ece4 100644
--- a/include/linux/rtc.h
+++ b/include/linux/rtc.h
@@ -228,6 +228,8 @@ extern int rtc_read_alarm(struct rtc_device *rtc,
 			struct rtc_wkalrm *alrm);
 extern int rtc_set_alarm(struct rtc_device *rtc,
 				struct rtc_wkalrm *alrm);
+extern int rtc_initialize_alarm(struct rtc_device *rtc,
+				struct rtc_wkalrm *alrm);
 extern void rtc_update_irq(struct rtc_device *rtc,
 			unsigned long num, unsigned long events);
 


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