diff mbox series

[V2,12/17] timers: Silently ignore timers with a NULL function

Message ID 20221122173648.793640919@linutronix.de
State Superseded
Headers show
Series timers: Provide timer_shutdown[_sync]() | expand

Commit Message

Thomas Gleixner Nov. 22, 2022, 5:45 p.m. UTC 
Tearing down timers which have circular dependencies to other
functionality, e.g. workqueues, where the timer can schedule work and work
can arm timers is not trivial.

In those cases it is desired to shutdown the timer in a way which prevents
rearming of the timer. The mechanism to do so it to set timer->function to
NULL and use this as an indicator for the timer arming functions to ignore
the (re)arm request.

In preparation for that replace the warnings in the relevant code pathes
with checks for timer->function == NULL and discard the rearm request
silently.

Add debug_assert_init() instead of the WARN_ON_ONCE(!timer->function)
checks so that debug objects can warn about non-initialized timers.

If developers fail to enable debug objects and then waste lots of time to
figure out why their non-initialized timer is not firing, they deserve it.

Co-developed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/20220407161745.7d6754b3@gandalf.local.home
Link: https://lore.kernel.org/all/20221110064101.429013735@goodmis.org
---
V2: Use continue instead of return and amend the return value docs (Steven)
---
 kernel/time/timer.c |   60 +++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 55 insertions(+), 5 deletions(-)

Comments

Anna-Maria Behnsen Nov. 23, 2022, 11:06 a.m. UTC | #1 
On Tue, 22 Nov 2022, Thomas Gleixner wrote:

> Tearing down timers which have circular dependencies to other
> functionality, e.g. workqueues, where the timer can schedule work and work
> can arm timers is not trivial.
> 
> In those cases it is desired to shutdown the timer in a way which prevents
> rearming of the timer. The mechanism to do so it to set timer->function to
> NULL and use this as an indicator for the timer arming functions to ignore
> the (re)arm request.
> 
> In preparation for that replace the warnings in the relevant code pathes
> with checks for timer->function == NULL and discard the rearm request
> silently.
> 
> Add debug_assert_init() instead of the WARN_ON_ONCE(!timer->function)
> checks so that debug objects can warn about non-initialized timers.

Could you expand this paragraph, so that is is not missleading when a
reader is not aware of the details of debug objects? Otherwise it seems to
the reader that debug objects will warn when timer->function == NULL.

  The warning of debug objects does not cover the original
  WARN_ON_ONCE(!timer->function). It warns when timer was not initialized
  using timer_setup[_on_stack]() or via DEFINE_TIMER().


> If developers fail to enable debug objects and then waste lots of time to
> figure out why their non-initialized timer is not firing, they deserve it.


Thanks,

	Anna-Maria
Thomas Gleixner Nov. 23, 2022, 5:08 p.m. UTC | #2 
On Wed, Nov 23 2022 at 12:06, Anna-Maria Behnsen wrote:
> On Tue, 22 Nov 2022, Thomas Gleixner wrote:
>> Add debug_assert_init() instead of the WARN_ON_ONCE(!timer->function)
>> checks so that debug objects can warn about non-initialized timers.
>
> Could you expand this paragraph, so that is is not missleading when a
> reader is not aware of the details of debug objects? Otherwise it seems to
> the reader that debug objects will warn when timer->function == NULL.
>
>   The warning of debug objects does not cover the original
>   WARN_ON_ONCE(!timer->function). It warns when timer was not initialized
>   using timer_setup[_on_stack]() or via DEFINE_TIMER().

Good point.
diff mbox series

Patch

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1017,7 +1017,7 @@  static inline int
 	unsigned int idx = UINT_MAX;
 	int ret = 0;
 
-	BUG_ON(!timer->function);
+	debug_assert_init(timer);
 
 	/*
 	 * This is a common optimization triggered by the networking code - if
@@ -1044,6 +1044,14 @@  static inline int
 		 * dequeue/enqueue dance.
 		 */
 		base = lock_timer_base(timer, &flags);
+		/*
+		 * Has @timer been shutdown? This needs to be evaluated
+		 * while holding base lock to prevent a race against the
+		 * shutdown code.
+		 */
+		if (!timer->function)
+			goto out_unlock;
+
 		forward_timer_base(base);
 
 		if (timer_pending(timer) && (options & MOD_TIMER_REDUCE) &&
@@ -1070,6 +1078,14 @@  static inline int
 		}
 	} else {
 		base = lock_timer_base(timer, &flags);
+		/*
+		 * Has @timer been shutdown? This needs to be evaluated
+		 * while holding base lock to prevent a race against the
+		 * shutdown code.
+		 */
+		if (!timer->function)
+			goto out_unlock;
+
 		forward_timer_base(base);
 	}
 
@@ -1128,8 +1144,12 @@  static inline int
  * mod_timer_pending() is the same for pending timers as mod_timer(), but
  * will not activate inactive timers.
  *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
+ *
  * Return:
- * * %0 - The timer was inactive and not modified
+ * * %0 - The timer was inactive and not modified or was is in
+ *	  shutdown state and the operation was discarded
  * * %1 - The timer was active and requeued to expire at @expires
  */
 int mod_timer_pending(struct timer_list *timer, unsigned long expires)
@@ -1155,8 +1175,12 @@  EXPORT_SYMBOL(mod_timer_pending);
  * same timer, then mod_timer() is the only safe way to modify the timeout,
  * since add_timer() cannot modify an already running timer.
  *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded, the return value is 0 and meaningless.
+ *
  * Return:
- * * %0 - The timer was inactive and started
+ * * %0 - The timer was inactive and started or was is in shutdown
+ *	  state and the operation was discarded
  * * %1 - The timer was active and requeued to expire at @expires or
  *	  the timer was active and not modified because @expires did
  *	  not change the effective expiry time
@@ -1176,8 +1200,12 @@  EXPORT_SYMBOL(mod_timer);
  * modify an enqueued timer if that would reduce the expiration time. If
  * @timer is not enqueued it starts the timer.
  *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
+ *
  * Return:
- * * %0 - The timer was inactive and started
+ * * %0 - The timer was inactive and started or was is in shutdown
+ *	  state and the operation was discarded
  * * %1 - The timer was active and requeued to expire at @expires or
  *	  the timer was active and not modified because @expires
  *	  did not change the effective expiry time such that the
@@ -1202,6 +1230,9 @@  EXPORT_SYMBOL(timer_reduce);
  *
  * If @timer->expires is already in the past @timer will be queued to
  * expire at the next timer tick.
+ *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
  */
 void add_timer(struct timer_list *timer)
 {
@@ -1218,13 +1249,18 @@  EXPORT_SYMBOL(add_timer);
  *
  * This can only operate on an inactive timer. Attempts to invoke this on
  * an active timer are rejected with a warning.
+ *
+ * If @timer->function == NULL then the start operation is silently
+ * discarded.
  */
 void add_timer_on(struct timer_list *timer, int cpu)
 {
 	struct timer_base *new_base, *base;
 	unsigned long flags;
 
-	if (WARN_ON_ONCE(timer_pending(timer) || !timer->function))
+	debug_assert_init(timer);
+
+	if (WARN_ON_ONCE(timer_pending(timer)))
 		return;
 
 	new_base = get_timer_cpu_base(timer->flags, cpu);
@@ -1235,6 +1271,13 @@  void add_timer_on(struct timer_list *tim
 	 * wrong base locked.  See lock_timer_base().
 	 */
 	base = lock_timer_base(timer, &flags);
+	/*
+	 * Has @timer been shutdown? This needs to be evaluated while
+	 * holding base lock to prevent a race against the shutdown code.
+	 */
+	if (!timer->function)
+		goto out_unlock;
+
 	if (base != new_base) {
 		timer->flags |= TIMER_MIGRATING;
 
@@ -1248,6 +1291,7 @@  void add_timer_on(struct timer_list *tim
 
 	debug_timer_activate(timer);
 	internal_add_timer(base, timer);
+out_unlock:
 	raw_spin_unlock_irqrestore(&base->lock, flags);
 }
 EXPORT_SYMBOL_GPL(add_timer_on);
@@ -1537,6 +1581,12 @@  static void expire_timers(struct timer_b
 
 		fn = timer->function;
 
+		if (WARN_ON_ONCE(!fn)) {
+			/* Should never happen. Emphasis on should! */
+			base->running_timer = NULL;
+			continue;
+		}
+
 		if (timer->flags & TIMER_IRQSAFE) {
 			raw_spin_unlock(&base->lock);
 			call_timer_fn(timer, fn, baseclk);