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.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 14 insertions, 5 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index da6e10c7a378..933efa4071c3 100644
--- a/kernel/time/timekeeping.c
+++ b/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);