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

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Date:	Fri, 27 Apr 2007 11:03:39 +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,
	David Howells <dhowells@...hat.com>
Subject: Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work

On Fri, Apr 27, 2007 at 11:52:47AM +0400, Oleg Nesterov wrote:
> On 04/27, Jarek Poplawski wrote:
...
> > > Sorry, can't understand. done == 0 means that the queueing in progress,
> > > this work should be placed on cwq->worklist very soon, most probably
> > > right after we drop cwq->lock.
> > 
> > I think, theoretically, probably, maybe, there is possible some strange
> > case, this function gets spin_lock only when: list_empty(&work->entry) == 1
> > && _PENDING == 1 && del_timer(&dwork->timer) == 0.
> 
> Yes, but this is not so strange, this means the queueing in progress. Most
> probably the "owner" of WORK_STRUCT_PENDING bit spins waiting for cwq->lock.
> We will retry in this case. Of course, if we have a workqueue with the single
> work which just re-arms itself via queue_work() (without delay) and does nothing
> more, we may need a lot of looping.

I've forgot most of the math already, but there is (probably)
some Parkinson's Law about it. So, by this strange case I
mean really lot of looping (something around infinity - quite
precisely).

> 
> > PS: probably unusable, but for my own satisfaction:
> > 
> > Acked-by: Jarek Poplawski <jarkao2@...pl>
> 
> It is useable, at least for me. I hope you will re-ack when I actually send

This is even more strange...
BTW, I take a week of vacation (people here deserve to rest
from me), so let's say it's both acked and re-acked by me.

> the patch. Note that the "else" branch above doesn't need cwq->lock, and we
> should start with del_timer(), because the pending timer is the most common
> case.

I see, you've thought about it probably more than you said
so, I trust you 100% here (but will check later, anyway...).

Cheers,
Jarek P. 
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