clocksource: Add module refcount

Add a module refcount, so the current clocksource cannot be removed
unconditionally.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Magnus Damm <magnus.damm@gmail.com>
Link: http://lkml.kernel.org/r/20130425143435.762417789@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/clocksource.h

@@ -21,6 +21,7 @@
 /* clocksource cycle base type */
 typedef u64 cycle_t;
 struct clocksource;
+struct module;
 
 #ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
 #include <asm/clocksource.h>
@@ -162,6 +163,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
  * @suspend:		suspend function for the clocksource, if necessary
  * @resume:		resume function for the clocksource, if necessary
  * @cycle_last:		most recent cycle counter value seen by ::read()
+ * @owner:		module reference, must be set by clocksource in modules
  */
 struct clocksource {
 	/*
@@ -195,6 +197,7 @@ struct clocksource {
 	cycle_t cs_last;
 	cycle_t wd_last;
 #endif
+	struct module *owner;
 } ____cacheline_aligned;
 
 /*

kernel/time/timekeeping.c

@@ -627,11 +627,20 @@ static int change_clocksource(void *data)
 	write_seqcount_begin(&timekeeper_seq);
 
 	timekeeping_forward_now(tk);
-	if (!new->enable || new->enable(new) == 0) {
-		old = tk->clock;
-		tk_setup_internals(tk, new);
-		if (old->disable)
-			old->disable(old);
+	/*
+	 * If the cs is in module, get a module reference. Succeeds
+	 * for built-in code (owner == NULL) as well.
+	 */
+	if (try_module_get(new->owner)) {
+		if (!new->enable || new->enable(new) == 0) {
+			old = tk->clock;
+			tk_setup_internals(tk, new);
+			if (old->disable)
+				old->disable(old);
+			module_put(old->owner);
+		} else {
+			module_put(new->owner);
+		}
 	}
 	timekeeping_update(tk, true, true);