linux-kernel - Re: [PATCH, RFC] sysfs: only allow one scheduled removal callback per kobj

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Message-ID: <20090311184729.110761e4@gondolin>
Date:	Wed, 11 Mar 2009 18:47:29 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Greg KH <gregkh@...e.de>
Cc:	Alex Chiang <achiang@...com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Ingo Molnar <mingo@...e.hu>, jbarnes@...tuousgeek.org,
	tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH, RFC] sysfs: only allow one scheduled removal callback
 per kobj

On Wed, 11 Mar 2009 08:32:28 -0700,
Greg KH <gregkh@...e.de> wrote:

> Why can't you use device_unregister()?  Or, you could use device_del(),
> which lets you rely on the fact that the device structure is still
> around for a bit, but it will disappear from sysfs.  Just don't forget
> to do the final put_device() on it to free the memory and "really"
> release it.
> 
> Or am I missing something else here?

You can't unregister a device from one of its attribute callbacks, it
locks up in sysfs (removing the sysfs dir waits for all active
references to be dropped, but the reference obtained before calling
->store won't be dropped until after ->store returned...)
device_schedule_callback() was introduced to solve exactly that problem.

(For the original oops, I'd rather solve the problem by making sure the
caller doesn't trigger removal several times - should probably be less
code than the proposed patch?)
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