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

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Message-ID: <20070423090030.GC1684@ff.dom.local>
Date:	Mon, 23 Apr 2007 11:00:30 +0200
From:	Jarek Poplawski <jarkao2@...pl>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work

On Fri, Apr 20, 2007 at 09:08:36PM +0400, Oleg Nesterov wrote:
> On 04/20, Jarek Poplawski wrote:
> >
> > On Thu, Apr 19, 2007 at 02:21:22PM +0400, Oleg Nesterov wrote:
> > ...
> > > Yes. It would be better to use cancel_work_sync() instead of flush_workqueue()
> > > to make this less possible (because cancel_work_sync() doesn't need to wait for
> > > the whole ->worklist), but we can't.
> > > 
> > > > Maybe this patch could check, if I'm not dreaming...
> > > 
> > > Also: cancel_rearming_delayed_work() will hang if it (or cancel_delayed_work())
> > > was already called.
> > > 
> > > I had some ideas how to make this interface reliable, but I can't see how to do
> > > this without uglification of the current code.
> > 
> > For some time I thought about using a flag (isn't there
> > one available after NOAUTOREL?), e.g. WORK_STRUCT_CANCEL,
> > as a sign:
> > 
> > - for a workqueue code: that the work shouldn't be queued,
> > nor executed, if possiblei, at first possible check.
> 
> Well, yes and no, afaics. (note also that NOAUTOREL has already gone).

I thought I wrote the same (sorry for my English)... 

> 
> First, this flag should be cleared after return from cancel_rearming_delayed_work().

I think this flag, if at all, probably should be cleared only
consciously by the owner of a work, maybe as a schedule_xxx_work
parameter, (but shouldn't be used from work handlers for rearming).
Mostly it should mean: we are closing (and have no time to chase
our work)...

> Also, we should add a lot of nasty checks to workqueue.c

Checking a flag isn't nasty - it's clear. IMHO current way of checking,
whether cancel succeeded, is nasty.

> 
> I _think_ we can re-use WORK_STRUCT_PENDING to improve this interface.
> Note that if we set WORK_STRUCT_PENDING, the work can't be queued, and
> dwork->timer can't be started. The only problem is that it is not so
> trivial to avoid races.

If there were no place, it would be better, then current way.
But WORK_STRUCT_PENDING couldn't be used for some error checking,
as it's now.

> 
> I'll try to do something on Sunday.
> 
> > - for a work function: to stop execution as soon as possible,
> > even without completing the usual job, at first possible check.
> 
> I doubt we need this "in general". It is easy to add some flag to the
> work_struct's container and check it in work->func() when needed.

Yes, but currently you cannot to behave like this e.g. with
"rearming" work. And maybe a common api could save some work.
But of course, if you have better way to assure this, it's OK
with me and congratulations!

Regards,
Jarek P.
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