linux-kernel - Re: refcounting drivers' data structures used in sysfs buffers

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Message-Id: <200703120954.22038.oneukum@suse.de>
Date:	Mon, 12 Mar 2007 09:54:21 +0100
From:	Oliver Neukum <oneukum@...e.de>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Maneesh Soni <maneesh@...ibm.com>, gregkh@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: refcounting drivers' data structures used in sysfs buffers

Am Samstag, 10. März 2007 20:19 schrieb Alan Stern:
> On Fri, 9 Mar 2007, Oliver Neukum wrote:
> 
> > Am Freitag, 9. März 2007 21:08 schrieb Alan Stern:
> > > After some more thought, I basically agree with what Oliver wrote
> > > originally.  sysfs_dirent is indeed the logical place to store the kref
> > > pointer.  However it needs to be used during open and release, not during
> > 
> > OK.
> > 
> > > read, write, and poll.  Another point, which Oliver didn't think of, is
> > > that the kref pointer needs to be passed to the driver as an argument in
> > > the show() and store() method calls.
> > 
> > Why? What's wrong with simply calling kref_get/put?
> 
> It's the same old problem: the race between unbind and sysfs I/O.  What
> good does holding a reference to the private data structure do if the
> show/store method gets called after the driver has been unbound from the
> device?  dev_get_drvdata() will no longer provide a valid pointer to the
> private data, so the method will have no way to access it.  Hence the
> method needs another argument.

It does half the job. You can make sure the driver is not asked to access
freed memory.
It is true that a driver will have to mark that device "disconnected"
and return errors if that device's attributes are referenced, but this can
be done internally.

Yes, this is a bit more complicated.
{rant mode}
Who came up with the idea of making life simpler by adding a code path?
All these problems were already solved for device nodes. Ioctl is ugly, but
at least a known code path.
{rant off}

> (BTW, the sysfs core would actually need more than a kref.  It would also 
> need a pointer to a release routine -- the kref contains only the atomic

Yes, this is implied.

	Regards
		Oliver
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