PM / Wakeup: Combine atomic counters to avoid reordering issues

The memory barrier in wakeup_source_deactivate() is supposed to
prevent the callers of pm_wakeup_pending() and pm_get_wakeup_count()
from seeing the new value of events_in_progress (0, in particular)
and the old value of event_count at the same time.  However, if
wakeup_source_deactivate() is executed by CPU0 and, for instance,
pm_wakeup_pending() is executed by CPU1, where both processors can
reorder operations, the memory barrier in wakeup_source_deactivate()
doesn't affect CPU1 which can reorder reads.  In that case CPU1 may
very well decide to fetch event_count before it's modified and
events_in_progress after it's been updated, so pm_wakeup_pending()
may fail to detect a wakeup event.  This issue can be addressed by
using a single atomic variable to store both events_in_progress
and event_count, so that they can be updated together in a single
atomic operation.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/wakeup.c

@@ -24,12 +24,26 @@
  */
 bool events_check_enabled;
 
-/* The counter of registered wakeup events. */
-static atomic_t event_count = ATOMIC_INIT(0);
-/* A preserved old value of event_count. */
+/*
+ * Combined counters of registered wakeup events and wakeup events in progress.
+ * They need to be modified together atomically, so it's better to use one
+ * atomic variable to hold them both.
+ */
+static atomic_t combined_event_count = ATOMIC_INIT(0);
+
+#define IN_PROGRESS_BITS	(sizeof(int) * 4)
+#define MAX_IN_PROGRESS		((1 << IN_PROGRESS_BITS) - 1)
+
+static void split_counters(unsigned int *cnt, unsigned int *inpr)
+{
+	unsigned int comb = atomic_read(&combined_event_count);
+
+	*cnt = (comb >> IN_PROGRESS_BITS);
+	*inpr = comb & MAX_IN_PROGRESS;
+}
+
+/* A preserved old value of the events counter. */
 static unsigned int saved_count;
-/* The counter of wakeup events being processed. */
-static atomic_t events_in_progress = ATOMIC_INIT(0);
 
 static DEFINE_SPINLOCK(events_lock);
 
@@ -307,7 +321,8 @@ static void wakeup_source_activate(struct wakeup_source *ws)
 	ws->timer_expires = jiffies;
 	ws->last_time = ktime_get();
 
-	atomic_inc(&events_in_progress);
+	/* Increment the counter of events in progress. */
+	atomic_inc(&combined_event_count);
 }
 
 /**
@@ -394,14 +409,10 @@ static void wakeup_source_deactivate(struct wakeup_source *ws)
 	del_timer(&ws->timer);
 
 	/*
-	 * event_count has to be incremented before events_in_progress is
-	 * modified, so that the callers of pm_check_wakeup_events() and
-	 * pm_save_wakeup_count() don't see the old value of event_count and
-	 * events_in_progress equal to zero at the same time.
+	 * Increment the counter of registered wakeup events and decrement the
+	 * couter of wakeup events in progress simultaneously.
 	 */
-	atomic_inc(&event_count);
-	smp_mb__before_atomic_dec();
-	atomic_dec(&events_in_progress);
+	atomic_add(MAX_IN_PROGRESS, &combined_event_count);
 }
 
 /**
@@ -556,8 +567,10 @@ bool pm_wakeup_pending(void)
 
 	spin_lock_irqsave(&events_lock, flags);
 	if (events_check_enabled) {
-		ret = ((unsigned int)atomic_read(&event_count) != saved_count)
-			|| atomic_read(&events_in_progress);
+		unsigned int cnt, inpr;
+
+		split_counters(&cnt, &inpr);
+		ret = (cnt != saved_count || inpr > 0);
 		events_check_enabled = !ret;
 	}
 	spin_unlock_irqrestore(&events_lock, flags);
@@ -579,19 +592,22 @@ bool pm_wakeup_pending(void)
  */
 bool pm_get_wakeup_count(unsigned int *count)
 {
-	bool ret;
+	unsigned int cnt, inpr;
 
 	if (capable(CAP_SYS_ADMIN))
 		events_check_enabled = false;
 
-	while (atomic_read(&events_in_progress) && !signal_pending(current)) {
+	for (;;) {
+		split_counters(&cnt, &inpr);
+		if (inpr == 0 || signal_pending(current))
+			break;
 		pm_wakeup_update_hit_counts();
 		schedule_timeout_interruptible(msecs_to_jiffies(TIMEOUT));
 	}
 
-	ret = !atomic_read(&events_in_progress);
-	*count = atomic_read(&event_count);
-	return ret;
+	split_counters(&cnt, &inpr);
+	*count = cnt;
+	return !inpr;
 }
 
 /**
@@ -605,11 +621,12 @@ bool pm_get_wakeup_count(unsigned int *count)
  */
 bool pm_save_wakeup_count(unsigned int count)
 {
+	unsigned int cnt, inpr;
 	bool ret = false;
 
 	spin_lock_irq(&events_lock);
-	if (count == (unsigned int)atomic_read(&event_count)
-	    && !atomic_read(&events_in_progress)) {
+	split_counters(&cnt, &inpr);
+	if (cnt == count && inpr == 0) {
 		saved_count = count;
 		events_check_enabled = true;
 		ret = true;