linux-kernel - Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed

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Date:	Wed, 25 Apr 2007 18:47:59 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Jarek Poplawski <jarkao2@...pl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	David Howells <dhowells@...hat.com>
Subject: Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work

On 04/25, Oleg Nesterov wrote:
>
> On 04/25, Jarek Poplawski wrote:
> >
> >                   Probably this is also possible without timer i.e.
> >  with queue_work.
> 
> Yes, thanks. While adding cpu-hotplug check I forgot to add ->current_work
> check, which is needed to actually implement this
> 
> 	> > Note that cancel_rearming_delayed_work() now can handle the works
> 	> > which re-arm itself via queue_work(), not only queue_delayed_work().
> 
> part. I'll resend after fix.

Hm. But can't we do better? Looks like we don't need to check ->current_work,

	void cancel_rearming_delayed_work(struct delayed_work *dwork)
	{
		struct work_struct *work = &dwork->work;
		struct cpu_workqueue_struct *cwq = get_wq_data(work);
		int done;

		do {
			done = 1;
			spin_lock_irq(&cwq->lock);

			if (!list_empty(&work->entry))
				list_del_init(&work->entry);
			else if (test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
				done = del_timer(&dwork->timer)

			spin_unlock_irq(&cwq->lock);
		} while (!done);

		/*
		 * Nobody can clear WORK_STRUCT_PENDING. This means that the
		 * work can't be re-queued and the timer can't be re-started.
		 */
		needs_a_good_name(cwq->wq, work);
		work_clear_pending(work);
	}

Jarek, I didn't think much about this, just a new idea. I am posting this code
in a hope you can review it while I sleep on this... CPU-hotplug is ignored for
now. Note that this version doesn't need the change in run_workqueue().

Oleg.

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