linux-kernel - Re: [PATCH v2 04/23] counter: Provide a wrapper to access device private data

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Message-ID: <93232592-f8df-2413-25c4-da697493eb84@metafoo.de>
Date:   Mon, 27 Dec 2021 13:28:42 +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
Subject: Re: [PATCH v2 04/23] counter: Provide a wrapper to access device
 private data

On 12/27/21 12:52 PM, Greg Kroah-Hartman wrote:
> On Mon, Dec 27, 2021 at 12:34:28PM +0100, Lars-Peter Clausen wrote:
>> On 12/27/21 12:02 PM, Greg Kroah-Hartman wrote:
>>> On Mon, Dec 27, 2021 at 10:45:07AM +0100, Uwe Kleine-König wrote:
>>>> For now this just wraps accessing struct counter_device::priv. However
>>>> this is about to change and converting drivers to this helper
>>>> individually makes fixing device lifetime issues result in easier to
>>>> review patches.
>>>>
>>>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
>>>> ---
>>>>    drivers/counter/counter-core.c | 12 ++++++++++++
>>>>    include/linux/counter.h        |  2 ++
>>>>    2 files changed, 14 insertions(+)
>>>>
>>>> diff --git a/drivers/counter/counter-core.c b/drivers/counter/counter-core.c
>>>> index f053a43c6c04..00c41f28c101 100644
>>>> --- a/drivers/counter/counter-core.c
>>>> +++ b/drivers/counter/counter-core.c
>>>> @@ -45,6 +45,18 @@ static struct bus_type counter_bus_type = {
>>>>    static dev_t counter_devt;
>>>> +/**
>>>> + * counter_priv - access counter device private data
>>>> + * @counter: counter device
>>>> + *
>>>> + * Get the counter device private data
>>>> + */
>>>> +void *counter_priv(const struct counter_device *const counter)
>>>> +{
>>>> +	return counter->priv;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(counter_priv);
>>> Shouldn't this be usin gdev_get_drvdata() and using the private data
>>> pointer that is already on the struct device structure itself?  The void
>>> *priv; should be dropped from struct counter_device entirely.
>>>
>>> Oh ick, I just now looked at 'struct counter_device', there are other
>>> reference counting issues in there (hint, struct cdev has a reference
>>> count...)  But that's independent of this patch series...
>> This is not a problem. The struct cdev holds a reference to the struct dev.
>> This allows them to use the same allocation. As long as there is a reference
>> to the cdev there will be a reference to the dev and the memory will be kept
>> alive.
> Ick, a cdev shouldn't be doing stuff like that, but I see how people
> like to use it that way :(
>
Can you explain why cdev shouldn't be doing that? This commit has some 
backstory why this is done 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=233ed09d7fdacf592ee91e6c97ce5f4364fbe7c0