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

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Message-ID: <20070426164436.GB1933@tv-sign.ru>
Date:	Thu, 26 Apr 2007 20:44:36 +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,
	David Howells <dhowells@...hat.com>
Subject: Re: Fw: [PATCH -mm] workqueue: debug possible endless loop in cancel_rearming_delayed_work

On 04/26, Jarek Poplawski wrote:
>
> On Wed, Apr 25, 2007 at 04:47:14PM +0400, Oleg Nesterov wrote:
> ...
> > > > > +		spin_lock_irq(&cwq->lock);
> > > > > +		/* CPU_DEAD in progress may change cwq */
> > > > > +		if (likely(cwq == get_wq_data(work))) {
> > > > > +			list_del_init(&work->entry);
> > > > > +			__set_bit(WORK_STRUCT_PENDING, work_data_bits(work));
> > > > > +			retry = try_to_del_timer_sync(&dwork->timer) < 0;
> > > > > +		}
> > > > > +		spin_unlock_irq(&cwq->lock);
> > > > > +	} while (unlikely(retry));
> > 
> > >  1. If delayed_work_timer_fn of this work is fired and is waiting
> > >  on the above spin_lock then, after above spin_unlock, the work
> > >  will be queued.
> > 
> > No, in that case try_to_del_timer_sync() returns -1.
> 
> Yes. But I think it's safe only after moving work_clear_pending
> in run_workqueue under a lock; probably otherwise there is a
> possibility this flag could be cleared, after above unlock.  

It doesn't matter in this particular case because we are going to retry
anyway. But yes, this patch moves work_clear_pending() under lock, because
otherwise it could be cleared by run_workqueue() if this work is about
to be executed, but was already deleted from list.

Oleg.

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