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

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Message-ID: <20070420170836.GB470@tv-sign.ru>
Date:	Fri, 20 Apr 2007 21:08:36 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Jarek Poplawski <jarkao2@...pl>
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 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).

First, this flag should be cleared after return from cancel_rearming_delayed_work().
Also, we should add a lot of nasty checks to workqueue.c

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.

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.

Oleg.

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