linux-kernel - Re: [linux-pm] [PATCH 2.6.25-rc1] cpufreq: fix cpufreq policy refcount imbalance

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Date:	Fri, 15 Feb 2008 10:24:21 -0800
From:	Greg KH <gregkh@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Yi Yang <yi.y.yang@...el.com>, akpm@...ux-foundation.org,
	davej@...emonkey.org.uk, cpufreq@...ts.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	mingo@...e.hu, torvalds@...ux-foundation.org,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [linux-pm] [PATCH 2.6.25-rc1] cpufreq: fix cpufreq policy
	refcount imbalance

On Fri, Feb 15, 2008 at 10:52:51AM -0500, Alan Stern wrote:
> On Fri, 15 Feb 2008, Yi Yang wrote:
> 
> > This patch adds kobject_put to balance refcount. I noticed Greg suggests
> > it will fix a power-off issue to remove kobject_get statement block, but i
> > think that isn't the best way because those code block has existed very long
> > and it is helpful because the successive statements are invoking relevant
> > data.
> 
> Are you referring to this section of code (before the region affected 
> by your patch)?
> 
> 	if (!kobject_get(&data->kobj)) {
> 		spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
> 		cpufreq_debug_enable_ratelimit();
> 		unlock_policy_rwsem_write(cpu);
> 		return -EFAULT;
> 	}
> 
> Greg is correct that the kobject_get() here is useless and should be
> removed.  kobject_get() never returns NULL unless its argument is NULL.  
> Since &data->kobj can never be NULL, the "if" test will never fail.  
> Hence there's no point in making the test at all.
> 
> The fact that a section of code has existed for a long time doesn't 
> mean that it is right.  :-)
> 
> Furthermore, there's no reason to do the kobject_get().  Holding 2 
> references to a kobject is no better than holding just 1 reference.  
> Assuming you know that the kobject is still registered, then you also 
> know that there is already a reference to it.  So you have no reason to 
> take an additional reference.

There's the additional problem that this second reference count is never
dropped, causing a bug :)

thanks,

greg k-h
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