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

Anna-Maria Behnsen
2022-11-23T10:52:46Z

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.
>
> 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(-)
>
> --- 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)
> {

Could you move the new paragraph after the paragraph where is is mentioned,
that timer->function has to be set prior calling add_timer()?

Thanks,

Anna-Maria