linux-kernel - Re: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed

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Date:	Fri, 20 Apr 2007 10:13:26 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	David Chinner <dgc@....com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work

On Fri, Apr 20, 2007 at 12:46:18AM +1000, David Chinner wrote:
> On Thu, Apr 19, 2007 at 08:54:04AM +0200, Jarek Poplawski wrote:
> > Hi,
> > 
> > IMHO cancel_rearming_delayed_work is dangerous place:
> 
> Agreed - I spent a couple of hours today learning why it
> can only be used on work functions that always rearm...
> 
> > - it assumes a work function always rearms (with no exception),
> > which probably isn't explained enough now (but anyway should
> > be checked in such loops);
> >
> > - probably possible (theoretical) scenario: a few work
> > functions rearm themselves with very short, equal times;
> > before flush_workqueue ends, their timers are already
> > fired, so cancel_delayed_work has nothing to do.
> 
> Easier than that - have a work function that rearms only if there's
> more work to do in the future. You only arm the timer when you
> have work to do, and it only rearms if there's more work to
> do in the future (e.g. rotating expiry lists).
> 
> i.e. while there's more work to do, you need to call
> cancel_rearming_delayed_work() to stop it reliably, but if you race
> with the work function not restarting itself, you hang.....

I'm not sure I correctly get your point, but according to
this comment:

" * cancel_rearming_delayed_work - reliably kill off a delayed
keventd work whose handler rearms the delayed work."

there is a question, whether a function that "rearms only if"
- "rearms". It seems the author of this comment didn't think
so and it was obvious to him/her cancel_rearming_delayed_work
wasn't intended for this case. At first I thought it's only a
language question - now, I see it's probably logical, too.

Cheers,
Jarek P.
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