linux-kernel - Re: [PATCH v5 1/5] counter: Internalize sysfs interface code

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Date:   Tue, 20 Oct 2020 10:38:43 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     jic23@...nel.org, kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v5 1/5] counter: Internalize sysfs interface code

On 10/18/20 9:49 AM, William Breathitt Gray wrote:
> On Mon, Oct 12, 2020 at 09:15:00PM -0500, David Lechner wrote:
>> On 9/26/20 9:18 PM, William Breathitt Gray wrote:
>>> This is a reimplementation of the Generic Counter driver interface.
>>
>> I'll follow up if I find any problems while testing but here are some
>> comments I had from looking over the patch.
>>
>>> diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
>>> new file mode 100644
>>> index 000000000000..987c6e8277eb
>>> --- /dev/null
>>> +++ b/drivers/counter/counter-core.c
>>
>>
>>> +/**
>>> + * counter_register - register Counter to the system
>>> + * @counter:	pointer to Counter to register
>>> + *
>>> + * This function registers a Counter to the system. A sysfs "counter" directory
>>> + * will be created and populated with sysfs attributes correlating with the
>>> + * Counter Signals, Synapses, and Counts respectively.
>>> + */
>>> +int counter_register(struct counter_device *const counter)
>>> +{
>>> +	struct device *const dev = &counter->dev;
>>> +	int err;
>>> +
>>> +	/* Acquire unique ID */
>>> +	counter->id = ida_simple_get(&counter_ida, 0, 0, GFP_KERNEL);
>>> +	if (counter->id < 0)
>>> +		return counter->id;
>>> +
>>> +	/* Configure device structure for Counter */
>>> +	dev->type = &counter_device_type;
>>> +	dev->bus = &counter_bus_type;
>>> +	if (counter->parent) {
>>> +		dev->parent = counter->parent;
>>> +		dev->of_node = counter->parent->of_node;
>>> +	}
>>> +	dev_set_name(dev, "counter%d", counter->id);
>>> +	device_initialize(dev);> +	dev_set_drvdata(dev, counter);
>>> +
>>> +	/* Add Counter sysfs attributes */
>>> +	err = counter_sysfs_add(counter);
>>> +	if (err)
>>> +		goto err_free_id;
>>> +
>>> +	/* Add device to system */
>>> +	err = device_add(dev);
>>> +	if (err) {
>>> +		put_device(dev);
>>> +		goto err_free_id;
>>> +	}
>>> +
>>> +	return 0;
>>> +
>>> +err_free_id:
>>> +	/* get_device/put_device combo used to free managed resources */
>>> +	get_device(dev);
>>> +	put_device(dev);
>>
>> I've never seen this in a driver before, so it makes me think this is
>> not the "right way" to do this. After device_initialize() is called, we
>> already should have a reference to dev, so only device_put() is needed.
> 
> I do admit this feels very hacky, but I'm not sure how to handle this
> more elegantly. The problem is that device_initialize() is not enough by
> itself -- it just initializes the structure, while device_add()
> increments the reference count via a call to get_device().

add_device() also releases this reference by calling put_device() in all
return paths. The reference count is initialized to 1 in device_initialize().

device_initialize > kobject_init > kobject_init_internal > kref_init

> 
> So let's assume counter_sysfs_add() fails and we go to err_free_id
> before device_add() is called; if we ignore get_device(), the reference
> count will be 0 at this point. We then call put_device(), which calls
> kobject_put(), kref_put(), and eventually refcount_dec_and_test().
> 
> The refcount_dec_and_test() function returns 0 at this point because the
> reference count can't be decremented further (refcount is already 0), so
> release() is never called and we end up leaking all the memory we
> allocated in counter_sysfs_add().
> 
> Is my logic correct here?

Not quite. I think you missed a few things I mentioned above. So we never
have a ref count of 0 until the very last call to put_device().