linux-kernel - Re: [RFC] Input: Remove unsafe device module references

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Date:	Tue, 1 Nov 2011 10:52:10 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Greg KH <gregkh@...e.de>
Cc:	David Herrmann <dh.herrmann@...glemail.com>,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] Input: Remove unsafe device module references

On Tue, Nov 01, 2011 at 10:01:56AM -0700, Greg KH wrote:
> On Tue, Nov 01, 2011 at 04:41:40PM +0100, David Herrmann wrote:
> > Hi Dmitry and Greg
> > 
> > It doesn't make sense to take a reference to our own module. When we call
> > module_put(THIS_MODULE) we cannot make sure that our module is still alive when
> > this function returns. Therefore, module_put() will return to invalid memory and
> > our input_dev_release() function is no longer available.
> > 
> > It would be interesting if Greg could elaborate what else we could do to replace
> > this module-refcount as it is definitely needed here. However, "struct device"
> > doesn't provide an owner field so there is no way for us to let the device core
> > keep a reference to our module.
> 
> For a bus module, yes, this is needed, so don't remove these calls, it's
> wrong to do so.

Strictly speaking, David is right, there is a race condition here.
However since we do module_put() as very last operation of
input_dev_release() it is extremely hard to trigger this race.

Until we have a better way of pinning the bus (or class) implementation
in memory we should keep __module_get/module_put in input core.

-- 
Dmitry
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