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

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Message-ID: <CANeycqqO06+u6Qy3zL=G_a=_sTYDOzUYf8wu7B7RGFJfuGNoRg@mail.gmail.com>
Date:	Fri, 7 Dec 2012 04:16:34 -0800
From:	Wedson Almeida Filho <wedsonaf@...il.com>
To:	Ming Lei <ming.lei@...onical.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

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

Right. device_del() puts the reference count of the parent -- is it
guaranteed that device_del() won't ever reassign dev->parent though
(e.g., to NULL)? I don't think it is, so I think that patch should
also save the pointer to the parent and use it (instead of what
happens to be in than dev->parent) to release the lock and put the
ref.

> 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.

Yeah, the pattern of locking the parent followed by the device occurs
in a few places. It looks like they were added by Alan with commit
bf74ad5bc41727d5f2f1c6bedb2c1fac394de731. (And as Greg mentioned,
might be occurring often enough to merit being moved into a common
function.)

I guess the question is whether the callee is allowed to call
device_move(), if not, we're good.

> 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.

How would that help?
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