linux-kernel - Re: [PATCH] make-cancel_rearming_delayed

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Date:	Mon, 7 May 2007 15:55:11 +0400
From:	Anton Vorontsov <cbou@...l.ru>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Jarek Poplawski <jarkao2@...pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Chinner <dgc@....com>,
	David Howells <dhowells@...hat.com>,
	Gautham Shenoy <ego@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] make-cancel_rearming_delayed_work-reliable-fix

On Mon, May 07, 2007 at 02:34:20PM +0400, Oleg Nesterov wrote:
> On 05/07, Jarek Poplawski wrote:
> >
> > There is a lot of new things in the final version of this
> > patch. I guess, there was no such problem in the previous
> > version.
> 
> No, this is basically the same patch + re-check-cwq-after-lock,
> the latter is mostly needed to prevent racing with CPU-hotplug.
> 
> > I can also see you have new doubts about usefulness, which
> > I cannot understand:
> > - even if there are some slowdowns, where does it matter?
> > - the "old" method uses only one method of cancelling, i.e.
> > del_timer, not trying to stop requeuing or to remove from
> > the queue; it seems to be effective only with long delayed
> > timers, and its real problems are probably mostly invisible.
> 
> The slowdown is small, changelog mentions it just to be "fair".
> 
> I am not happy with the complication this patch adds, mostly
> I hate this smb_wmb() in insert_work(). I have an idea how to
> remove it later, but this needs another patch not related to
> workqueue.c.
> 
> > BTW, I'm still not convinced all additions are needed:
> > the "old" cancel_rearming_  doesn't care about checking
> > or waiting on anything after del_timer positive.
> 
> It would be very strange to do wait_on_work() only in case
> when del_timer() failed. This way we still need to do
> cancel_work_sync() after cancel_rearming_delayed_work(),
> but only when del_timer() failed, ugly. Note also that
> wait_on_work() does not sleep if work->func() is not running.
> 
> Also, consider this callback:
> 
> 	void work_handler(struct work_struct *w)
> 	{
> 		struct delayed_work dw = container_of(...);
> 
> 		queue_delayed_work(dw, delay);
> 
> 		// <------------- cancel_rearming_delayed_work()
> 
> 		cancel_delayed_work(dw);
> 		queue_delayed_work(dw, another_delay);
> 	}
> 
> Yes, this is strange and ugly. But correct! The current version

I guess pseudo code below is not that strange, but real usecase:

probe()
{
	INIT_DELAYED_WORK(...);
	/* we're not issuing queue_delayed_work() in probe(), work will
	 * be started by interrupt */
	return;
}

remove()
{
	/* hang will happen here if there was no queue_delyed_work()
	 * call (like there was no interrupts, which starts rearming
	 * work */
	cancel_rearming_delayed_work();
}


Your patch will fix it, right?

> Oleg.

Good luck,

-- 
Anton Vorontsov
email: cbou@...l.ru
backup email: ya-cbou@...dex.ru
irc://irc.freenode.org/bd2
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