linux-kernel - Re: [linux-pm] [PATCH v4] pm_qos: make update

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Message-ID: <20100611174954.41df0606@schatten.dmk.lab>
Date:	Fri, 11 Jun 2010 17:49:54 +0200
From:	Florian Mickler <florian@...kler.org>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Jonathan Corbet <corbet@....net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	markgross@...gnar.org, linville@...driver.com,
	linux-kernel@...r.kernel.org,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [linux-pm] [PATCH v4] pm_qos: make update_request non blocking

On Fri, 11 Jun 2010 09:25:52 -0500
James Bottomley <James.Bottomley@...e.de> wrote:

> On Thu, 2010-06-10 at 16:41 +0200, Florian Mickler wrote:
> > > > So the notified value is always the latest or there is another
> > > > notification underway.
> > > 
> > > Well, no ... it's a race, and like all good races the winner is non
> > > deterministic.
> > 
> > Can you point out where I'm wrong?
> > 
> > U1. update_request gets called
> > U2. 	new extreme value gets calculated under spinlock
> > U3.	notify gets queued if its WORK_PENDING_BIT is not set.
> > 
> > run_workqueue() does the following:
> > R1. clears the WORK_PENDING_BIT
> > R2. calls update_notify()
> > R3. 	reads the current extreme value
> > R4. 	notification gets called with that value		
> > 
> > 
> > If another update_request comes to schedule_work before
> > run_workqueue() has cleared the WORK_PENDING_BIT, the work will not be
> > requeued, but R3 isn't yet executed. So the notifiers will get the last
> > value.
> 
> So the race now only causes lost older notifications ... as long as the
> consumers are OK with that (it is an API change) then this should work.
> You're still not taking advantage of the user context passed in, though,
> so this does needlessly delay notifications for that case.

Right. We can use execute_in_process_context. 
 
> Actually, pm_qos_remove now needs a flush_scheduled work since you don't
> want to return until the list is clear (since the next action may be to
> free the object).

Yes. Good point, will fix. 

> James
> 

Cheers,
Flo

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