linux-kernel - Re: [PATCH 2/2] enhance sysfs rfkill interface

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Message-ID: <20100312233925.74effc7d@schatten.dmk.lab>
Date:	Fri, 12 Mar 2010 23:39:25 +0100
From:	Florian Mickler <florian@...kler.org>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	"John W. Linville" <linville@...driver.com>,
	Marcel Holtmann <marcel@...tmann.org>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Johannes Berg <johannes@...solutions.net>,
	linux-wireless@...r.kernel.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Alan Jenkins <alan-jenkins@...fmail.co.uk>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/2] enhance sysfs rfkill interface

On Fri, 12 Mar 2010 13:20:26 -0800
Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:

> On Fri, Mar 12, 2010 at 09:57:43PM +0100, Florian Mickler wrote:
> > On Fri, 12 Mar 2010 10:22:09 -0800
> > Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> > 
> > > On Fri, Mar 12, 2010 at 07:03:08PM +0100, florian@...kler.org wrote:
> > > >  
> > > > +static ssize_t rfkill_hard_show(struct device *dev,
> > > > +				 struct device_attribute *attr,
> > > > +				 char *buf)
> > > > +{
> > > > +	struct rfkill *rfkill = to_rfkill(dev);
> > > > +	unsigned long flags;
> > > > +	u32 state;
> > > > +
> > > > +	spin_lock_irqsave(&rfkill->lock, flags);
> > > > +	state = rfkill->state;
> > > > +	spin_unlock_irqrestore(&rfkill->lock, flags);
> > > 
> > > Why exactly is this lock needed?
> > 
> > The rfkill state is updated from multiple contexts... Am I overlooking
> > smth obvious here?
> > 
> 
> You are not updating but reading... Are you concerned about seeing
> a partial write to u32? It does not happen.
> 
Hm.. You shure? On every arch that supports wireless drivers? 

I've just copied that code from the old sysfs state-file handler.
So I assumed that reading partial updated state can happen... Also I
just searched a little but did not find anything, cause i didn't know
where to look. Who garantees this? Is it a gcc thing? 

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