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

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Message-ID: <CACVXFVMOxQEJ-nrvGQ-PkevqKkLnXu+QjHgeswkbzFjqOVYCdQ@mail.gmail.com>
Date:	Fri, 7 Dec 2012 21:04:19 +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 8:16 PM, Wedson Almeida Filho <wedsonaf@...il.com> wrote:
>> 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.)

In fact, there is no shutdown callback defined for usb driver, so holding
the parent lock in device_shutdown() might be removed.

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

Not only the callee, and other contexts can change device->parent
too. Looks rfcomm_tty_open() which calls device_move() is called
in open() context, so the parent might be changed before unlock(dev->parent)
in __driver_attach().

>
>> 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?

device_pm_lock() can prevent device_move() from being running.

Thanks,
--
Ming Lei
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