linux-kernel - Re: [PATCH] make cancel_rearming_delayed

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Message-ID: <20070508143217.GA1011@ff.dom.local>
Date:	Tue, 8 May 2007 16:32:17 +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

On Tue, May 08, 2007 at 06:05:17PM +0400, Oleg Nesterov wrote:
> On 05/08, Jarek Poplawski wrote:
> >
> > On Tue, May 08, 2007 at 04:31:02PM +0400, Oleg Nesterov wrote:
> > > 
> > > I thought about adding such a parameter, and I don't like this. This is
> > > a matter of taste, of course, but _imho_ this uglifies the code.
> > > 
> > > In any case, unless we do completely different patch, the sequence should be
> > > 
> > > 	del_timer()	- a pending timer is the most common case
> > > 
> > > 	test_and_set_bit(WORK_STRUCT_PENDING) - the work is idle
> > > 
> > > 	try-to-steal-the-queued-work
> > > 
> > > This is what we are doing now.
> > 
> > I simply don't like to call del_timer(), where not needed, but maybe
> > it's not so expensive and we can afford it... 
> 
> But this is the most common case, that was my point.

And I agree it should start... 

> 
> Look, we can add
> 
> 	if (!get_wq_data(work))
> 		/* it was never queued */
> 		return;
> 
> at the very start of cancel_xxx() functions. This will optimize out
> del_timer + try_to_grab_pending() _sometimes_. Should we do this?
> I don't think so. I think it is better to save a couple of bytes
> from i-cache, but not to try to optimize the unlikely case.

But the most of our test is already done in test_and_set_bit.
I think it's more about taste, so let's forget...

> > > So, we should either return 0, or add BUG_ON(!cwq).
> > 
> > ...And you prefer endless loop. Seems brave!
> 
> No, no, the loop won't be endless. Why do you think so? We spin until the timer
> is started, or until the work is queued.

Probably you are right - I was afraid about some inactive
work with no timer to come.

So, I think this all looks fine, and maybe one more needless ack
won't do any harm here:

Acked-by: Jarek Poplawski <jarkao2@...pl>
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