Re: [PATCH] kthread: add kthread_mod_pending_delayed_work api

[Petr Mladek]

On Sun 2021-02-14 00:06:11, Yiwei Zhang wrote:
> The existing kthread_mod_delayed_work api will queue a new work if
> failing to cancel the current work due to no longer being pending.
> However, there's a case that the same work can be enqueued from both
> an async request and a delayed work, and a racing could happen if the
> async request comes right after the timeout delayed work gets scheduled,
> because the clean up work may not be safe to run twice.

Please, provide more details about the use case. Why the work is
originally sheduled with a delay. And and why it suddenly can/should
be proceed immediately.

> 
> Signed-off-by: Yiwei Zhang <zzyiwei@xxxxxxxxxxx>
> ---
>  include/linux/kthread.h |  3 +++
>  kernel/kthread.c        | 48 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+)
> 
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -1142,6 +1142,54 @@ bool kthread_mod_delayed_work(struct kthread_worker *worker,
>  }
>  EXPORT_SYMBOL_GPL(kthread_mod_delayed_work);
>  
> +/**
> + * kthread_mod_pending_delayed_work - modify delay of a pending delayed work
> + * @worker: kthread worker to use
> + * @dwork: kthread delayed work to queue
> + * @delay: number of jiffies to wait before queuing
> + *
> + * If @dwork is still pending modify @dwork's timer so that it expires after
> + * @delay. If @dwork is still pending and @delay is zero, @work is guaranteed to
> + * be queued immediately.
> + *
> + * Return: %true if @dwork was pending and its timer was modified,
> + * %false otherwise.
> + *
> + * A special case is when the work is being canceled in parallel.
> + * It might be caused either by the real kthread_cancel_delayed_work_sync()
> + * or yet another kthread_mod_delayed_work() call. We let the other command
> + * win and return %false here. The caller is supposed to synchronize these
> + * operations a reasonable way.
> + *
> + * This function is safe to call from any context including IRQ handler.
> + * See __kthread_cancel_work() and kthread_delayed_work_timer_fn()
> + * for details.
> + */
> +bool kthread_mod_pending_delayed_work(struct kthread_worker *worker,
> +				      struct kthread_delayed_work *dwork,
> +				      unsigned long delay)
> +{

kthread_worker API tries to follow the workqueue API. It helps to use and
switch between them easily.

workqueue API does not provide this possibility. Instead it has
flush_delayed_work(). It queues the work when it was pending and
waits until the work is procced. So, we might do:

bool kthread_flush_delayed_work(struct kthread_delayed_work *dwork)


> +	struct kthread_work *work = &dwork->work;
> +	unsigned long flags;
> +	int ret = true;
> +
> +	raw_spin_lock_irqsave(&worker->lock, flags);
> +	if (!work->worker || work->canceling ||
> +	    !__kthread_cancel_work(work, true, &flags)) {
> +		ret = false;
> +		goto out;
> +	}

Please, use separate checks with comments as it is done, for example,
in kthread_mod_delayed_work()

	struct kthread_work *work = &dwork->work;
	unsigned long flags;
	int ret;

	raw_spin_lock_irqsave(&worker->lock, flags);

	/* Do not bother with canceling when never queued. */
	if (!work->worker)
		goto nope;

	/* Do not fight with another command that is canceling this work. */
	if (work->canceling)
		goto nope;

	/* Nope when the work was not pending. */
	ret = __kthread_cancel_work(work, true, &flags);
	if (!ret)
		nope;

	/* Queue the work immediately. */
	kthread_insert_work(worker, work, &worker->work_list);
	raw_spin_unlock_irqrestore(&worker->lock, flags);

	return kthread_flush_work(work);
nope:
	raw_spin_unlock_irqrestore(&worker->lock, flags);
	return false;


Will this work for you?

Best Regards,
Petr