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

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Date:	Mon, 7 May 2007 12:31:08 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	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 Sun, May 06, 2007 at 01:32:13AM +0400, Oleg Nesterov wrote:
> on top of
> 	make-cancel_rearming_delayed_work-reliable-spelling.patch
> 
> Add cpu-relax() into spinloops, and a comments update.
> 
> Small note. The new implementation has another downside. Suppose that rearming
> work->func() gets a preemtion after setting WORK_STRUCT_PENDING but before
> add_timer/__queue_work. In that case cancel_rearming_delayed_work() will burn
> CPU in a busy-wait loop. Fortunately this can happen only with CONFIG_PREEMPT
> and we spin with preemption enabled.
> 
> We can avoid this,
> 
> 	void cancel_rearming_delayed_work(struct delayed_work *dwork)
> 	{
> 		int retry;
> 
> 		do {
> 			retry = !del_timer(&dwork->timer) &&
> 				!try_to_grab_pending(&dwork->work);
> 			wait_on_work(&dwork->work);
> 		} while (retry);
> 
> 		work_clear_pending(&dwork->work);
> 	}
> 
> but I don't think this is worth fixing.

I think so.

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.

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.

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.

Regards,
Jarek P.

PS: I'll try to check this all in the evening and will
write tomorrow, if found something interesting.
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