[RFC][patch 2/5] cleanup clocksource selection

[Martin Schwidefsky]

From: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>

Introduce clocksource_dequeue & clocksource_update and move spinlock
calls. clocksource_update does nothing for GENERIC_TIME=n since
change_clocksource does nothing as well.

Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: john stultz <johnstul@xxxxxxxxxx>
Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
---
 kernel/time/clocksource.c |  123 ++++++++++++++++++++++++++--------------------
 1 file changed, 70 insertions(+), 53 deletions(-)

Index: linux-2.6/kernel/time/clocksource.c
===================================================================
--- linux-2.6.orig/kernel/time/clocksource.c
+++ linux-2.6/kernel/time/clocksource.c
@@ -341,46 +341,25 @@ struct clocksource *clocksource_get_next
 	return curr_clocksource;
 }
 
-/**
- * select_clocksource - Selects the best registered clocksource.
- *
- * Private function. Must hold clocksource_lock when called.
- *
- * Select the clocksource with the best rating, or the clocksource,
- * which is selected by userspace override.
- */
-static struct clocksource *select_clocksource(void)
-{
-	struct clocksource *next;
-
-	if (list_empty(&clocksource_list))
-		return NULL;
-
-	if (clocksource_override)
-		next = clocksource_override;
-	else
-		next = list_entry(clocksource_list.next, struct clocksource,
-				  list);
-
-	if (next == curr_clocksource)
-		return NULL;
-
-	return next;
-}
-
 /*
  * Enqueue the clocksource sorted by rating
  */
 static int clocksource_enqueue(struct clocksource *c)
 {
 	struct list_head *tmp, *entry = &clocksource_list;
+	unsigned long flags;
+	int rc;
 
+	spin_lock_irqsave(&clocksource_lock, flags);
+	rc = 0;
 	list_for_each(tmp, &clocksource_list) {
 		struct clocksource *cs;
 
 		cs = list_entry(tmp, struct clocksource, list);
-		if (cs == c)
-			return -EBUSY;
+		if (cs == c) {
+			rc = -EBUSY;
+			goto out;
+		}
 		/* Keep track of the place, where to insert */
 		if (cs->rating >= c->rating)
 			entry = tmp;
@@ -390,8 +369,23 @@ static int clocksource_enqueue(struct cl
 	if (strlen(c->name) == strlen(override_name) &&
 	    !strcmp(c->name, override_name))
 		clocksource_override = c;
+out:
+	spin_unlock_irqrestore(&clocksource_lock, flags);
+	return rc;
+}
+
+/*
+ * Dequeue a clocksource
+ */
+static void clocksource_dequeue(struct clocksource *cs)
+{
+	unsigned long flags;
 
-	return 0;
+	spin_lock_irqsave(&clocksource_lock, flags);
+	list_del(&cs->list);
+	if (clocksource_override == cs)
+		clocksource_override = NULL;
+	spin_unlock_irqrestore(&clocksource_lock, flags);
 }
 
 /**
@@ -539,6 +533,25 @@ void clocksource_forward_now(void)
 }
 
 /**
+ * select_clocksource - Selects the best registered clocksource.
+ *
+ * Private function. Must hold clocksource_lock when called.
+ *
+ * Select the clocksource with the best rating, or the clocksource,
+ * which is selected by userspace override.
+ */
+static struct clocksource *select_clocksource(void)
+{
+	if (list_empty(&clocksource_list))
+		return NULL;
+
+	if (clocksource_override)
+		return clocksource_override;
+
+	return list_entry(clocksource_list.next, struct clocksource, list);
+}
+
+/**
  * change_clocksource - Swaps clocksources if a new one is available
  *
  * Accumulates current time interval and initializes new clocksource
@@ -577,6 +590,23 @@ void change_clocksource(void)
 	       clock->name);
 	 */
 }
+
+/**
+ * clocksource_update - Check if a better clocksource is available
+ */
+static void clocksource_update(void)
+{
+	struct clocksource *new;
+	unsigned long flags;
+
+	spin_lock_irqsave(&clocksource_lock, flags);
+	new = select_clocksource();
+	if (new)
+		next_clocksource = new;
+	spin_unlock_irqrestore(&clocksource_lock, flags);
+}
+#else /* CONFIG_GENERIC_TIME */
+static inline void clocksource_update(void) { }
 #endif
 
 /**
@@ -587,16 +617,13 @@ void change_clocksource(void)
  */
 int clocksource_register(struct clocksource *c)
 {
-	unsigned long flags;
 	int ret;
 
-	spin_lock_irqsave(&clocksource_lock, flags);
 	ret = clocksource_enqueue(c);
-	if (!ret)
-		next_clocksource = select_clocksource();
-	spin_unlock_irqrestore(&clocksource_lock, flags);
-	if (!ret)
+	if (!ret) {
+		clocksource_update();
 		clocksource_check_watchdog(c);
+	}
 	return ret;
 }
 EXPORT_SYMBOL(clocksource_register);
@@ -607,14 +634,10 @@ EXPORT_SYMBOL(clocksource_register);
  */
 void clocksource_change_rating(struct clocksource *cs, int rating)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(&clocksource_lock, flags);
-	list_del(&cs->list);
+	clocksource_dequeue(cs);
 	cs->rating = rating;
 	clocksource_enqueue(cs);
-	next_clocksource = select_clocksource();
-	spin_unlock_irqrestore(&clocksource_lock, flags);
+	clocksource_update();
 }
 
 /**
@@ -622,14 +645,8 @@ void clocksource_change_rating(struct cl
  */
 void clocksource_unregister(struct clocksource *cs)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(&clocksource_lock, flags);
-	list_del(&cs->list);
-	if (clocksource_override == cs)
-		clocksource_override = NULL;
-	next_clocksource = select_clocksource();
-	spin_unlock_irqrestore(&clocksource_lock, flags);
+	clocksource_dequeue(cs);
+	clocksource_update();
 }
 
 /**
@@ -721,13 +738,13 @@ static ssize_t sysfs_override_clocksourc
 	}
 
 	/* Reselect, when the override name has changed */
-	if (ovr != clocksource_override) {
+	if (ovr != clocksource_override)
 		clocksource_override = ovr;
-		next_clocksource = select_clocksource();
-	}
 
 	spin_unlock_irq(&clocksource_lock);
 
+	clocksource_update();
+
 	return ret;
 }
 

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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