RTC: assure proper memory ordering with respect to RTC_DEV_BUSY flag

We must make sure that the RTC_DEV_BUSY flag has proper lock semantics,
i.e.  that the RTC_DEV_BUSY stores clearing the flag don't get reordered
before the preceeding stores and loads and vice versa.

Spotted by Nick Piggin.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: David Brownell <david-b@pacbell.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/rtc/interface.c

@@ -293,7 +293,7 @@ int rtc_irq_register(struct rtc_device *rtc, struct rtc_task *task)
 		return -EINVAL;
 
 	/* Cannot register while the char dev is in use */
-	if (test_and_set_bit(RTC_DEV_BUSY, &rtc->flags))
+	if (test_and_set_bit_lock(RTC_DEV_BUSY, &rtc->flags))
 		return -EBUSY;
 
 	spin_lock_irq(&rtc->irq_task_lock);
@@ -303,7 +303,7 @@ int rtc_irq_register(struct rtc_device *rtc, struct rtc_task *task)
 	}
 	spin_unlock_irq(&rtc->irq_task_lock);
 
-	clear_bit(RTC_DEV_BUSY, &rtc->flags);
+	clear_bit_unlock(RTC_DEV_BUSY, &rtc->flags);
 
 	return retval;
 }

drivers/rtc/rtc-dev.c

@@ -26,7 +26,7 @@ static int rtc_dev_open(struct inode *inode, struct file *file)
 					struct rtc_device, char_dev);
 	const struct rtc_class_ops *ops = rtc->ops;
 
-	if (test_and_set_bit(RTC_DEV_BUSY, &rtc->flags))
+	if (test_and_set_bit_lock(RTC_DEV_BUSY, &rtc->flags))
 		return -EBUSY;
 
 	file->private_data = rtc;
@@ -41,7 +41,7 @@ static int rtc_dev_open(struct inode *inode, struct file *file)
 	}
 
 	/* something has gone wrong */
-	clear_bit(RTC_DEV_BUSY, &rtc->flags);
+	clear_bit_unlock(RTC_DEV_BUSY, &rtc->flags);
 	return err;
 }
 
@@ -402,7 +402,7 @@ static int rtc_dev_release(struct inode *inode, struct file *file)
 	if (rtc->ops->release)
 		rtc->ops->release(rtc->dev.parent);
 
-	clear_bit(RTC_DEV_BUSY, &rtc->flags);
+	clear_bit_unlock(RTC_DEV_BUSY, &rtc->flags);
 	return 0;
 }