Re: [PATCH 2/2] enhance sysfs rfkill interface

[Dmitry Torokhov]

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@xxxxxxxxx> 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@xxxxxxxxx> wrote:
> > > 
> > > > On Fri, Mar 12, 2010 at 07:03:08PM +0100, florian@xxxxxxxxxxx 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
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