linux-kernel - Re: [PATCH] cancel_delayed_work: use del_timer() instead of del_timer

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Date:	Thu, 26 Apr 2007 16:29:21 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cancel_delayed_work: use del_timer() instead of del_timer_sync()

On Wed, Apr 25, 2007 at 04:52:14PM +0400, Oleg Nesterov wrote:
> On 04/25, Jarek Poplawski wrote:
> >
> > On Wed, Apr 25, 2007 at 01:50:34AM +0400, Oleg Nesterov wrote:
> > > del_timer_sync() buys nothing for cancel_delayed_work(), but it is less
> > > efficient since it locks the timer unconditionally, and may wait for the
> > > completion of the delayed_work_timer_fn().
> > 
> > I'm not sure what is the main aim of this patch.
> 
> optimization
> 
> >                                                  It seems this
> > change cannot do any harm, but anyway it could change a few
> > things, e.g. with current version of cancel_rearming_delayed_work
> > some flush_workqueue could be done needlessly, before the work
> > is queued from timer.
> 
> I don't think so... Could you clarify?

With a code like:

if (!cancel_delayed_work(dwork))
	flush_workqueue(wq);
	
if cancel_ returns 0, and there is _queue_work in progress,
flush_ will be done once, after this work is queued.

After the patch, and the same situation flush_ also runs
one time, but maybe without the work in a queue.

So, if there is no more loops, there could be difference,
and even if very unprobable, something could stop working
after such change.

> 
> >                       It's not a big deal here, but if anybody
> > did something like this without loop - it could matter.
> > 
> > So, probably a lot of current code should be checked, before
> > applying and I doubt the gain is worth of this. Maybe, for
> > safety, make this with new name as an alternative and
> > deprecate the current version?
> 
> This change should not make any visible difference for the callers,
> otherwise it is buggy.

IMHO, there is the same visible difference,
as between del_timer and del_timer_sync.

Regards,
Jarek P.
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