linux-kernel - Re: [PATCH 1/3] pda_power: add support for writeable properties

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Date:	Wed, 12 May 2010 00:28:25 +0200
From:	Daniel Mack <daniel@...aq.de>
To:	Anton Vorontsov <cbouatmailru@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	David Woodhouse <dwmw2@...radead.org>,
	Alexey Starikovskiy <astarikovskiy@...e.de>,
	Len Brown <len.brown@...el.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Matt Reimer <mreimer@...p.net>,
	Evgeniy Polyakov <zbr@...emap.net>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/3] pda_power: add support for writeable properties

On Tue, May 11, 2010 at 10:23:47PM +0400, Anton Vorontsov wrote:
> On Tue, May 11, 2010 at 07:58:12PM +0200, Daniel Mack wrote:
> [...]
> > Hmm, no. The code defaults to (S_IRUSR | S_IRGRP | S_IROTH) which is
> > 0444, just like it was before. So there shouldn't be any regression. The
> > mode is only changed if the psy defines a property_is_writeable()
> > callback which returns 1. Or do I miss your point?
> 
> Yes, power_supply_attrs is a global array, and you shouldn't change
> it between power_supply_register() calls.
> 
> If you don't see why it's a bad idea in general, think about it
> other way, a race:
> 
> ...someone registers psy0 with attr X marked as read-only...
> ...code flow stops before device_create_file(psy0, global->mode)..
> [preempt]
> ...someone registers psy1 with attr X marked as writable...
> ...you set up global->mode to 0644...
> [preempt again]
> ...we end up calling device_create_file(psy0, 0644)...

Ah, I see. But the struct passed in is just used as template, right? So
for the particular case you outlined, a simple lock should do the trick,
right? I would prefer this over always-writeable file entries.

Daniel
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