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 <redacted>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/all/20220407161745.7d6754b3@gandalf.local.home (local)
Link: https://lore.kernel.org/all/20221110064101.429013735@goodmis.org (local)
---
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(-)

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c

@@ -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)
 {