Re: [PATCH 1/2] timers: WARN if add_timer_on is used with offlined cpu.

[Thomas Gleixner]

On Thu, Jan 16 2025 at 00:41, Imran Khan wrote:
> timer started on an offlined cpu will not fire after
> its expiry time and may never fire if that cpu remains
> offline.
> So add a WARN_ON_ONCE in add_timer_on, to indicate
> if any of its users are (wrongly) starting a timer
> on an offlined cpu.
>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
> ---
>  kernel/time/timer.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index a5860bf6d16f9..ec9eb58e45241 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1370,6 +1370,12 @@ void add_timer_on(struct timer_list *timer, int cpu)
>  	if (!timer->function)
>  		goto out_unlock;
>  
> +	/*
> +	 * WARN if specified cpu is offline, because on offlined cpu
> +	 * timer will not fire even after its expiry.
> +	 */
> +	WARN_ON_ONCE(!cpu_online(cpu));

Then why queueing the timer in the first place?

Thanks,

        tglx