linux-kernel - Re: [RFC PATCH 1/2] mac80211: make max_network

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Message-ID: <20100609122050.1dd18132@schatten.dmk.lab>
Date:	Wed, 9 Jun 2010 12:20:50 +0200
From:	Florian Mickler <florian@...kler.org>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	james.bottomley@...e.de, markgross@...gnar.org,
	mgross@...ux.intel.com,
	"John W. Linville" <linville@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	Javier Cardona <javier@...ybit.com>, Jouni Malinen <j@...fi>,
	Rui Paulo <rpaulo@...il.com>,
	Kalle Valo <kalle.valo@...ia.com>,
	linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 1/2] mac80211: make max_network_latency notifier
 atomic safe

On Wed, 09 Jun 2010 11:38:07 +0200
Johannes Berg <johannes@...solutions.net> wrote:

> On Wed, 2010-06-09 at 11:15 +0200, florian@...kler.org wrote:
> > In order to have the pm_qos framework be callable from interrupt
> > context, all listeners have to also be callable in that context.
> 
> That makes no sense at all. Why add work structs _everywhere_ in the
> callees and make the API harder to use and easy to get wrong completely,
> instead of just adding a single work struct that will be queued from the
> caller and dealing with the locking complexity etc. just once.
> 
> johannes

Just to defend this approach, but I'm certainly not married to it
(hence RFC):

There are only two listeners at the moment. I suspect that most future
uses of the framework need to be atomic, as the driver that
requests a specific quality of service probably doesn't want to get into
races with the provider of that service(listener). So i suspected the
network listener to be the special case.  

The race between service-provider and qos-requester for non-atomic
contextes is already there, isn't it? so, locking complexity shouldn't
be worse than before.

But my first approach to this is seen here:
https://lists.linux-foundation.org/pipermail/linux-pm/2010-June/026902.html

A third possibility would be to make it dependent on the
type of the constraint, if blocking notifiers are allowed or not. 
But that would sacrifice API consistency (update_request for one
constraint is allowed to be called in interrupt context and
update_request for another would be not).

Cheers,
Flo
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