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

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Date:	Thu, 26 Apr 2007 20:34:06 +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 04/26, Jarek Poplawski wrote:
>
> > 	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;
> 
> I don't understand, why you think cwq cannot be NULL here.

sure it can, this is just a template.

> > 
> > 		do {
> > 			done = 1;
> > 			spin_lock_irq(&cwq->lock);
> > 
> > 			if (!list_empty(&work->entry))
> > 				list_del_init(&work->entry);
> 
> BTW, isn't needs_a_good_name needles after this and after del_timer positive?

no, we still need it. work->func() may be running on another CPU as well.

> 
> > 			else if (test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
> > 				done = del_timer(&dwork->timer)
> 
> If this runs while a work function is fired in run_workqueue,
> it sets _PENDING bit, but if the work skips rearming, we have probably
> endless loop, again.

No, if the work skips rearming (or didn't yet), we set WORK_STRUCT_PENDING
successfully.

>                                  It is something alike to the current
> way, with some added measures: you try to shoot a work on the run,
> while queued or timer_pending, plus the _PENDING flag set, so it seems,
> there is some risk of longer than planed looping.

Sorry, can't understand. done == 0 means that the queueing in progress,
this work should be placed on cwq->worklist very soon, most probably
right after we drop cwq->lock.

> I have to look at this more, at home and, if something new, I'll write
> tomorrow. So, the good news, is you should have enough sleep this time!

Thanks for review!

Oleg.

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