Date: Wed, 8 Jul 2015 11:10:26 -0600
From: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
To: Jens Wiklander <jens.wiklander@linaro.org>
Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        devicetree@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rob Herring <robh+dt@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>, valentin.manea@huawei.com,
        jean-michel.delorme@st.com, emmanuel.michel@st.com, javier@javigon.com,
        Mark Rutland <mark.rutland@arm.com>,
        Michal Simek <michal.simek@xilinx.com>
Subject: Re: [PATCH v4 3/5] tee: generic TEE subsystem
Message-ID: <20150708171026.GA11740@obsidianresearch.com>
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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..

> +       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..

> +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...

> +	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.

Jason
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