linux-kernel - Re: [PATCH 4/4] workqueue: Remove now superfluous cancel_delayed

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Message-ID: <20110203164549.GA12802@core.coreip.homeip.net>
Date:	Thu, 3 Feb 2011 08:45:49 -0800
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Faisal Latif <faisal.latif@...el.com>,
	Roland Dreier <roland@...nel.org>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Alessandro Rubini <rubini@...vis.unipv.it>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	"David S. Miller" <davem@...emloft.net>,
	"John W. Linville" <linville@...driver.com>,
	Johannes Berg <johannes@...solutions.net>,
	Yong Zhang <yong.zhang0@...il.com>
Subject: Re: [PATCH 4/4] workqueue: Remove now superfluous
 cancel_delayed_work() calls

On Thu, Feb 03, 2011 at 05:19:06PM +0100, Tejun Heo wrote:
> Hello, Peter.
> 
> On Thu, Feb 03, 2011 at 03:09:44PM +0100, Peter Zijlstra wrote:
> > Since queue_delayed_work() can now deal with existing timers, we don't
> > need to explicitly call cancel_delayed_work() anymore.
> 
> This is nice but there's small complication with the way
> queue_delayed_work() behaves.  If a delayed work item is already
> pending, another queue_delayed_work() doesn't modify the delay whether
> the new delay is longer or shorter than the current one.  The previous
> patch doesn't really change the behavior as the whole thing is gated
> with WORK_STRUCT_PENDING_BIT.
> 
> So, cancel_delayed_work() followed by queue_delayed_work() schedules
> the work to be executed at the specified time regardless of the
> current pending state while queue_delayed_work() takes effect iff
> currently the work item is not pending.
> 
> The current behavior is weird and it often is easier to use explicit
> timer + work item if the timer needs to be modified, but it has been
> that way from the beginning so I don't think changing it would be a
> good idea.  We can introduce a new interface (mod_delayed_work()
> maybe) for this tho.
> 

I agree. If we were to change queue_delayed_work() we'd have to verify
that all users that do not presently use cancel_delayed_work() in
reschedule would be OK with the new behavior.

Thanks.

-- 
Dmitry
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