linux-kernel - Re: [PATCH v4 3/5] tee: generic TEE subsystem

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Date:	Thu, 9 Jul 2015 14:49:08 +0200
From:	Jens Wiklander <jens.wiklander@...aro.org>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	devicetree@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Rob Herring <robh+dt@...nel.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	valentin.manea@...wei.com, jean-michel.delorme@...com,
	emmanuel.michel@...com, javier@...igon.com,
	Mark Rutland <mark.rutland@....com>,
	Michal Simek <michal.simek@...inx.com>
Subject: Re: [PATCH v4 3/5] tee: generic TEE subsystem

On Wed, Jul 08, 2015 at 11:10:26AM -0600, Jason Gunthorpe wrote:
> On Wed, Jul 08, 2015 at 12:16:30PM +0200, Jens Wiklander wrote:
> 
> > +static void tee_device_complete_unused(struct kref *kref)
> > +{
> > +	struct tee_device *teedev;
> > +
> > +	teedev = container_of(kref, struct tee_device, users);
> > +	/* When the mutex is released, no other tee_device_get() will succeed */
> > +	teedev->desc = NULL;
> > +	complete(&teedev->c_no_users);
> > +}
> > +
> > +void tee_device_put(struct tee_device *teedev)
> > +{
> > +	mutex_lock(&teedev->mutex);
> > +	/* Shouldn't put in this state */
> > +	if (!WARN_ON(!teedev->desc))
> > +		kref_put(&teedev->users, tee_device_complete_unused);
> > +	mutex_unlock(&teedev->mutex);
> > +}
> > +
> > +bool tee_device_get(struct tee_device *teedev)
> > +{
> > +	mutex_lock(&teedev->mutex);
> > +	if (!teedev->desc) {
> > +		mutex_unlock(&teedev->mutex);
> > +		return false;
> > +	}
> > +	kref_get(&teedev->users);
> > +	mutex_unlock(&teedev->mutex);
> > +	return true;
> > +}
> 
> If you are holding the mutex then you don't really need a kref, just a
> simple active count counter.
> 
> I've been a bit learly lately about seeing krefs used for something
> other than kfree, I've seen a few subtle mistakes in those schemes -
> yours looks OK, only because of the lock, and the lock makes the kref
> redundant..

Thanks, I'll fix.

> 
> > +       cdev_init(&teedev->cdev, &tee_fops);
> > +       teedev->cdev.owner = teedesc->owner;
> 
> This also needs to set teedev->cdev.kobj.parent.
> I'm guessing:
> 
>  teedev->cdev.kobj.parent = &teedev->dev.kobj;
> 
> TPM had the same mistake..

OK. This triggered a discussion, from what I understand the outcome
is that what you're suggesting is the right thing to do here.

> 
> > +void tee_device_unregister(struct tee_device *teedev)
> > +{
> > +	if (IS_ERR_OR_NULL(teedev))
> > +		return;
> 
> See for some general colour on IS_ERR_OR_NULL
> 
> https://www.mail-archive.com/linux-omap@vger.kernel.org/msg78030.html
> 
> IMHO, you should never, store an ERR pointer into long term storage,
> so I wonder why this is like this...

OK, I'll replace the IS_ERR_OR_NULL with !teedev and only store a real
pointer or NULL here.

> 
> > +	if (teedev->flags & TEE_DEVICE_FLAG_REGISTERED) {
> > +		cdev_del(&teedev->cdev);
> > +		device_del(&teedev->dev);
> > +	}
> > +
> > +	tee_device_put(teedev);
> > +	wait_for_completion(&teedev->c_no_users);
> 
> Generally in a scheme like this we'd see open and release get/put the
> underlying module handle to prevent driver removal while the char dev
> is open. Otherwise module removal will hang here.

I'm perhaps misunderstanding you. While the cdev has any open file
descriptors rmmod will fail with "Resource temporarily unavailable"
because of fops_get() in chrdev_open().

tee_device_get()/put() deals with detaching of the driver, I see no
other way than blocking here once the detaching has been triggered.

--
Thanks,
Jens
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