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SubjectRe: [PATCH] kthread: add kthread_mod_pending_delayed_work api
On Fri, Feb 19, 2021 at 2:56 AM Petr Mladek <pmladek@suse.com> wrote:
>
> 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@android.com>
> > ---
> > 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

Thanks for your comments and reviews, Petr! I completely understand
Christoph's pushback regarding no upstream use case here. Just want to
see if this is a missing use case in kthread. I'll propose again if
later I find a use case in any upstream drivers.

Best,
Yiwei

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