Re: [PATCH] workqueue: warn if delayed_work is queued to an offlined cpu.

[Tejun Heo]

On Thu, Jan 09, 2025 at 09:18:29PM +1100, Imran Khan wrote:
> delayed_work submitted to an offlined cpu, will not get executed,
> after the specified delay if the cpu remains offline. If the cpu
> never comes online the work will never get executed.
> checking for online cpu in __queue_delayed_work, does not sound
> like a good idea because to do this reliably we need hotplug lock
> and since work may be submitted from atomic contexts, we would
> have to use cpus_read_trylock. But if trylock fails we would queue
> the work on any cpu and this may not be optimal because our intended
> cpu might still be online.
> 
> Putting a WARN_ON for an already offlined cpu, will indicate users
> of queue_delayed_work_on, if they are (wrongly) trying to queue
> delayed_work on offlined cpu.
> 
> Signed-off-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
> ---
>  kernel/workqueue.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 8e0bb3c608239..10878b5e3d74f 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2508,6 +2508,7 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
>  		return;
>  	}
>  
> +	WARN_ON(cpu != WORK_CPU_UNBOUND && !cpu_online(cpu));

Can we use WARN_ON_ONCE() instead?

Thanks.

-- 
tejun