timekeeping: Abort clocksource change in case of failure

There is no point to go through a full timekeeping update when acquiring a
module reference or enabling the new clocksource fails.

Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Anna-Maria Behnsen <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: John Stultz <[email protected]>
Link: https://lore.kernel.org/all/20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-4-554456a44a15@linutronix.de

kernel/time/timekeeping.c

@@ -1608,33 +1608,29 @@ static void __timekeeping_set_tai_offset(struct timekeeper *tk, s32 tai_offset)
 static int change_clocksource(void *data)
 {
 	struct timekeeper *tk = &tk_core.timekeeper;
-	struct clocksource *new, *old = NULL;
+	struct clocksource *new = data, *old = NULL;
 	unsigned long flags;
-	bool change = false;
-
-	new = (struct clocksource *) data;
 
 	/*
-	 * If the cs is in module, get a module reference. Succeeds
-	 * for built-in code (owner == NULL) as well.
+	 * If the clocksource is in a module, get a module reference.
+	 * Succeeds for built-in code (owner == NULL) as well. Abort if the
+	 * reference can't be acquired.
 	 */
-	if (try_module_get(new->owner)) {
-		if (!new->enable || new->enable(new) == 0)
-			change = true;
-		else
-			module_put(new->owner);
+	if (!try_module_get(new->owner))
+		return 0;
+
+	/* Abort if the device can't be enabled */
+	if (new->enable && new->enable(new) != 0) {
+		module_put(new->owner);
+		return 0;
 	}
 
 	raw_spin_lock_irqsave(&timekeeper_lock, flags);
 	write_seqcount_begin(&tk_core.seq);
 
 	timekeeping_forward_now(tk);
-
-	if (change) {
-		old = tk->tkr_mono.clock;
-		tk_setup_internals(tk, new);
-	}
-
+	old = tk->tkr_mono.clock;
+	tk_setup_internals(tk, new);
 	timekeeping_update(tk, TK_CLEAR_NTP | TK_MIRROR | TK_CLOCK_WAS_SET);
 
 	write_seqcount_end(&tk_core.seq);
@@ -1643,7 +1639,6 @@ static int change_clocksource(void *data)
 	if (old) {
 		if (old->disable)
 			old->disable(old);
-
 		module_put(old->owner);
 	}