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SubjectRe: [PATCH v2 16/23] counter: interrupt-cnt: Convert to new counter registration
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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@pengutronix.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.

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