Date: Fri, 12 Mar 2010 14:48:28 -0800
From: Dmitry Torokhov <dmitry.torokhov@gmail.com>
To: Florian Mickler <florian@mickler.org>
Cc: "John W. Linville" <linville@tuxdriver.com>,
	Marcel Holtmann <marcel@holtmann.org>,
	Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
	Johannes Berg <johannes@sipsolutions.net>,
	linux-wireless@vger.kernel.org,
	Randy Dunlap <rdunlap@xenotime.net>,
	Alan Jenkins <alan-jenkins@tuffmail.co.uk>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH 2/2] enhance sysfs rfkill interface
Message-ID: <20100312224828.GA18618@core.coreip.homeip.net>
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 <20100312182209.GB8736@core.coreip.homeip.net>
 <20100312215743.7671d173@schatten.dmk.lab>
 <20100312212025.GA17712@core.coreip.homeip.net>
 <20100312233925.74effc7d@schatten.dmk.lab>
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On Fri, Mar 12, 2010 at 11:39:25PM +0100, Florian Mickler wrote:
> On Fri, 12 Mar 2010 13:20:26 -0800
> Dmitry Torokhov <dmitry.torokhov@gmail.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@gmail.com> wrote:
> > > 
> > > > On Fri, Mar 12, 2010 at 07:03:08PM +0100, florian@mickler.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? 
> 

None of the arches would do byte-by-byte writes to a u32, they'd write
dword at once. Also, even if they could, you are interested in a single
flag (bit). You do realize that once you leave spinlock whatever you
fetched is stale data and may not be trusted?

-- 
Dmitry