Date: Sat, 13 Mar 2010 10:41:28 +0100
From: Florian Mickler <florian@mickler.org>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
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,
	Valdis.Kletnieks@vt.edu
Subject: Re: [PATCH 2/2] enhance sysfs rfkill interface
Message-ID: <20100313104128.79aa2846@schatten.dmk.lab>
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On Fri, 12 Mar 2010 14:48:28 -0800
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:

> 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?

On Fri, 12 Mar 2010 18:48:19 -0500
Valdis.Kletnieks@vt.edu wrote:
> If a u32 load or store from memory isn't atomic, the Linux kernel is screwed
> anyhow.  Hint - imagine if every 32-bit reference had to be treated the way
> we currently treat 64-bit references on a 32-bit system.


i presume, there is no way any digital device could write _one bit_
partial :) 
so this _may_ actually be safe *g* 
 
how about the write in the _store() function? there we
read,update and write back the whole 32 bit which then potentially
overwrites some other flag concurrently set by an driver interrupt on
another cpu? i think the lock there is needed.

cheers,
Flo