linux-kernel - Re: [PATCH 6/8] PM: Add suspend blocking work.

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Message-Id: <201004292309.06929.rjw@sisk.pl>
Date:	Thu, 29 Apr 2010 23:09:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Alan Stern <stern@...land.harvard.edu>,
	Tejun Heo <tj@...nel.org>, Pavel Machek <pavel@....cz>,
	Len Brown <len.brown@...el.com>
Subject: Re: [PATCH 6/8] PM: Add suspend blocking work.

On Thursday 29 April 2010, Arve Hjønnevåg wrote:
> 2010/4/28 Rafael J. Wysocki <rjw@...k.pl>:
> > On Thursday 29 April 2010, Arve Hjønnevåg wrote:
> >> 2010/4/28 Rafael J. Wysocki <rjw@...k.pl>:
> >> > On Wednesday 28 April 2010, Oleg Nesterov wrote:
> >> >> On 04/27, Arve Hjønnevåg wrote:
> >> >> >
> >> >> > Allow work to be queued that will block suspend while it is pending
> >> >> > or executing. To get the same functionality in the calling code often
> >> >> > requires a separate suspend_blocker for pending and executing work, or
> >> >> > additional state and locking. This implementation does add additional
> >> >> > state and locking, but this can be removed later if we add support for
> >> >> > suspend blocking work to the core workqueue code.
> >> >>
> >> >> I think this patch is fine.
> >> >>
> >> >> Just one silly question,
> >> >>
> >> >> > +int queue_suspend_blocking_work(struct workqueue_struct *wq,
> >> >> > +                           struct suspend_blocking_work *work)
> >> >> > +{
> >> >> > +   int ret;
> >> >> > +   unsigned long flags;
> >> >> > +
> >> >> > +   spin_lock_irqsave(&work->lock, flags);
> >> >> > +   suspend_block(&work->suspend_blocker);
> >> >> > +   ret = queue_work(wq, &work->work);
> >> >> > +   if (ret)
> >> >> > +           work->active++;
> >> >>
> >> >> why not
> >> >>
> >> >>       ret = queue_work(wq, &work->work);
> >> >>       if (ret) {
> >> >>               suspend_block(&work->suspend_blocker);
> >> >>               work->active++;
> >> >>       }
> >> >>
> >> >> ?
> >> >>
> >> >> Afaics, we can't race with work->func() doing unblock, we hold work-lock.
> >> >> And this way the code looks more clear.
> >> >
> >> > Agreed.  Arve, any objections to doing that?
> >> >
> >>
> >> I need to fix the race, but I can easily fix it in
> >> cancel_suspend_blocking_work_sync instead. If the suspend blocker is
> >> active for a long time, and DEBUG_SUSPEND_BLOCKER is enabled, we can
> >> tell if the work is constantly re-queued or if the workqueue is stuck.
> >
> > Well, perhaps that's worth adding a comment to the code.  The debug part is not
> > immediately visible from the code itself.
> 
> On second thought, this only makes a difference if both conditions are
> true. If we are constantly re-queuing the work but it is not stuck,
> either method will show the debug message, so I used Oleg's
> suggestion.

OK, great.

Rafael
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