linux-kernel - Re: Race condition between driver_probe_device and device

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Date:	Fri, 7 Dec 2012 11:42:47 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Wedson Almeida Filho <wedsonaf@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Greg KH <gregkh@...uxfoundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Linux PM List <linux-pm@...r.kernel.org>
Subject: Re: Race condition between driver_probe_device and device_shutdown

On Fri, Dec 7, 2012 at 8:09 AM, Wedson Almeida Filho <wedsonaf@...il.com> wrote:
> I don't have the setup anymore, I'll give it a shot if I ever get back it back.
>
> I have a question though: why do we need to get a reference to the
> parent? If the assumption is that the callee can release its reference

Because device_del() will put reference count of the parent, and the patch
only focuses on race between probe/release and shutdown.

> to the parent, then one may also expect callees to reassign
> dev->parent (e.g., by calling device_move), so it wouldn't be safe to
> later call device_unlock on the potentially different dev->parent.

As far as device_move() concerned, looks it might be a problem.
The problem even exits on driver attach vs. device open/release,
if device_move is called in open() and open() happens before driver
attach completes.

> If we expect dev->parent to remain unchanged, then there's no need to
> get an extra reference to the parent as the device itself already
> holds one (and we hold an extra one on the device).

As I said above, device_del() will put reference count of parent, but I forget
why the parent's lock has to be held in this patch.

> What am I missing?

Your concern on device_remove() might be correct. Also, I am wondering
if we can walk the 'dpm_list' backwards for device shutdown, which should
be simpler and more reasonable.

Thanks,
--
Ming Lei
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