linux-kernel - Re: [PATCH v2 16/23] counter: interrupt-cnt: Convert to new counter registration

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Message-ID: <c9300c02-2704-4f46-9e1f-74665e6b6f8b@metafoo.de>
Date:   Mon, 27 Dec 2021 12:44:29 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        kernel@...gutronix.de,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        Oleksij Rempel <linux@...pel-privat.de>
Subject: Re: [PATCH v2 16/23] counter: interrupt-cnt: Convert to new counter
 registration

On 12/27/21 12:34 PM, Greg Kroah-Hartman wrote:
> On Mon, Dec 27, 2021 at 12:21:14PM +0100, Lars-Peter Clausen wrote:
>> On 12/27/21 11:59 AM, Greg Kroah-Hartman wrote:
>>> On Mon, Dec 27, 2021 at 10:45:19AM +0100, Uwe Kleine-König wrote:
>>>> This fixes device lifetime issues where it was possible to free a live
>>>> struct device.
>>>>
>>>> Fixes: a55ebd47f21f ("counter: add IRQ or GPIO based counter")
>>>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>>>> ---
>>>>    drivers/counter/interrupt-cnt.c | 28 ++++++++++++++++------------
>>>>    1 file changed, 16 insertions(+), 12 deletions(-)
>>>>
>>>> diff --git a/drivers/counter/interrupt-cnt.c b/drivers/counter/interrupt-cnt.c
>>>> index 4bf706ef46e2..9e99702470c2 100644
>>>> --- a/drivers/counter/interrupt-cnt.c
>>>> +++ b/drivers/counter/interrupt-cnt.c
>>>> @@ -16,7 +16,6 @@
>>>>    struct interrupt_cnt_priv {
>>>>    	atomic_t count;
>>>> -	struct counter_device counter;
>>>>    	struct gpio_desc *gpio;
>>>>    	int irq;
>>>>    	bool enabled;
>>>> @@ -148,12 +147,14 @@ static const struct counter_ops interrupt_cnt_ops = {
>>>>    static int interrupt_cnt_probe(struct platform_device *pdev)
>>>>    {
>>>>    	struct device *dev = &pdev->dev;
>>>> +	struct counter_device *counter;
>>>>    	struct interrupt_cnt_priv *priv;
>>>>    	int ret;
>>>> -	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
>>>> -	if (!priv)
>>>> +	counter = devm_counter_alloc(dev, sizeof(*priv));
>>> I just picked one of these patches at random, nothing specific about
>>> this driver...
>>>
>>> You can not have a 'struct device' in memory allocated by devm_*()
>>> functions for the obvious reason that now that memory is being
>>> controlled by a reference count that is OUTSIDE of the structure itself.
>>>
>>> So while your goal might be good here, this is not the correct solution
>>> at all, sorry.
>> Before this patch the memory for the struct device was devm_kzalloc'ed.
>> Which as you point out is a bug.
>>
>> After this patch the memory is reference counted and will be freed when the
>> last reference is dropped, in the release callback of the struct device.
>>
>> The alloc function is still a devm_ function, but on 'free' it will only
>> drop the reference to the struct device that it holds. This is a very common
>> pattern that is used by basically any driver subsystem in the kernel.
> Then it is not a real devm_() call, let's not call it that please as it
> is obviously very confusing :)
>
> Just call it counter_alloc(), or , counter_create(), or something a bit
> more in line with the rest of all driver subsystems.

But all the other driver subsystems call this kind of function devm_... :)

Usually for everything you call in probe() you need a corresponding 
action in remove(). With the devm_... function is remove action will be 
automatically called.