Re: [patch V3 12/17] timers: Silently ignore timers with a NULL function
From: Anna-Maria Behnsen
Date: Thu Nov 24 2022 - 02:37:45 EST
On Wed, 23 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 is 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 paths
> with checks for timer->function == NULL. If the pointer is NULLL, then
s/NULLL/NULL
> 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.
>
> The warning of debug objects does warn if timer->function == NULL. It
does NOT warn
> 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. Same for initializing a timer with a NULL function.
>
> Co-developed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Reviewed-by: Jacob Keller <jacob.e.keller@xxxxxxxxx>
> Link: https://lore.kernel.org/all/20220407161745.7d6754b3@xxxxxxxxxxxxxxxxxx
> Link: https://lore.kernel.org/all/20221110064101.429013735@xxxxxxxxxxx
> ---
> V2: Use continue instead of return and amend the return value docs (Steven)
> V3: Changelog and comment updates (Anna-Maria)
> ---
> kernel/time/timer.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 52 insertions(+), 5 deletions(-)
>
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -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
You forgot to update this "was is" mistake. All other places are fine.
> * * %1 - The timer was active and requeued to expire at @expires
> */
> int mod_timer_pending(struct timer_list *timer, unsigned long expires)
Thanks,
Anna-Maria