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

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Date:	Wed, 28 Apr 2010 20:47:14 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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.

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.

-- 
Arve Hjønnevåg
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