From: Ingo Molnar <mingo@elte.hu>,
      john stultz <johnstul@us.ibm.com>

clocksource_lock is used in softirq-context via e.g.
timeofday_periodic_hook() -> get_next_clocksource(), but the lock is not
acquired in a softirq-safe manner - which could lead to deadlocks.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: john stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 kernel/time/clocksource.c |   44 ++++++++++++++++++++++--------------
 1 files changed, 27 insertions(+), 17 deletions(-)

diff -puN kernel/time/clocksource.c~time-clocksource-infrastructure-fix-clocksource_lock-deadlock kernel/time/clocksource.c
--- devel/kernel/time/clocksource.c~time-clocksource-infrastructure-fix-clocksource_lock-deadlock	2006-01-31 20:19:28.000000000 -0800
+++ devel-akpm/kernel/time/clocksource.c	2006-01-31 20:19:28.000000000 -0800
@@ -56,12 +56,14 @@ static char override_name[32];
  */
 struct clocksource *get_next_clocksource(void)
 {
-	spin_lock(&clocksource_lock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&clocksource_lock, flags);
 	if (next_clocksource) {
 		curr_clocksource = next_clocksource;
 		next_clocksource = NULL;
 	}
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irqrestore(&clocksource_lock, flags);
 
 	return curr_clocksource;
 }
@@ -70,6 +72,10 @@ struct clocksource *get_next_clocksource
  * select_clocksource - Finds the best registered clocksource.
  *
  * Private function. Must hold clocksource_lock when called.
+ *
+ * Looks through the list of registered clocksources, returning
+ * the one with the highest rating value. If there is a clocksource
+ * name that matches the override string, return that clocksource.
  */
 static struct clocksource *select_clocksource(void)
 {
@@ -127,18 +133,20 @@ static int is_registered_source(struct c
  */
 void register_clocksource(struct clocksource *c)
 {
-	spin_lock(&clocksource_lock);
+	unsigned long flags;
 
+	spin_lock_irqsave(&clocksource_lock, flags);
 	/* check if clocksource is already registered */
 	if (is_registered_source(c)) {
-		printk("register_clocksource: Cannot register %s. Already registered!",
+		printk("register_clocksource(%s): already registered!",
 		       c->name);
 	} else {
-		list_add(&c->list, &clocksource_list);
-		/* select next clocksource */
+		/* register it */
+ 		list_add(&c->list, &clocksource_list);
+		/* scan the registered clocksources, and pick the best one */
 		next_clocksource = select_clocksource();
 	}
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irqrestore(&clocksource_lock, flags);
 }
 
 EXPORT_SYMBOL(register_clocksource);
@@ -152,9 +160,11 @@ EXPORT_SYMBOL(register_clocksource);
  */
 void reselect_clocksource(void)
 {
-	spin_lock(&clocksource_lock);
+	unsigned long flags;
+
+	spin_lock_irqsave(&clocksource_lock, flags);
 	next_clocksource = select_clocksource();
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irqrestore(&clocksource_lock, flags);
 }
 
 /**
@@ -169,9 +179,9 @@ sysfs_show_current_clocksources(struct s
 {
 	char *curr = buf;
 
-	spin_lock(&clocksource_lock);
+	spin_lock_irq(&clocksource_lock);
 	curr += sprintf(curr, "%s ", curr_clocksource->name);
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irq(&clocksource_lock);
 
 	curr += sprintf(curr, "\n");
 
@@ -200,7 +210,7 @@ static ssize_t sysfs_override_clocksourc
 	if (count < 1)
 		return -EINVAL;
 
-	spin_lock(&clocksource_lock);
+	spin_lock_irq(&clocksource_lock);
 
 	/* copy the name given: */
 	memcpy(override_name, buf, count);
@@ -209,7 +219,7 @@ static ssize_t sysfs_override_clocksourc
 	/* try to select it: */
 	next_clocksource = select_clocksource();
 
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irq(&clocksource_lock);
 
 	return count;
 }
@@ -227,14 +237,14 @@ sysfs_show_available_clocksources(struct
 	struct list_head *tmp;
 	char *curr = buf;
 
-	spin_lock(&clocksource_lock);
+	spin_lock_irq(&clocksource_lock);
 	list_for_each(tmp, &clocksource_list) {
 		struct clocksource *src;
 
 		src = list_entry(tmp, struct clocksource, list);
 		curr += sprintf(curr, "%s ", src->name);
 	}
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irq(&clocksource_lock);
 
 	curr += sprintf(curr, "\n");
 
@@ -287,10 +297,10 @@ device_initcall(init_clocksource_sysfs);
  */
 static int __init boot_override_clocksource(char* str)
 {
-	spin_lock(&clocksource_lock);
+	spin_lock_irq(&clocksource_lock);
 	if (str)
 		strlcpy(override_name, str, sizeof(override_name));
-	spin_unlock(&clocksource_lock);
+	spin_unlock_irq(&clocksource_lock);
 	return 1;
 }
 
_