Re: [PATCH] iio: fix kobject_put warning in iio_device_register

[Zeng Heng <zengheng4@huawei.com>]

On 2022/11/11 20:05, Zeng Heng wrote:
>
> On 2022/11/11 19:31, Jonathan Cameron wrote:
>> On Thu, 10 Nov 2022 21:26:15 +0800
>> Zeng Heng <zengheng4@huawei.com> wrote:
>>
>>> There is warning reported by kobject lib in kobject_put():
>>>
>>> kobject: '(null)' (00000000be81a546): is not initialized, yet 
>>> kobject_put() is being called.
>>> WARNING: CPU: 0 PID: 535 at lib/kobject.c:718 kobject_put+0x12c/0x180
>>> Call Trace:
>>>   cdev_device_add
>>>   __iio_device_register
>>>   __devm_iio_device_register
>>>   tmp117_probe
>>>
>>> If don't need to register chardev for most of IIO devices,
>>> we just register them with device_add() only, and use device_del()
>>> to unregister them.
>>
>>
>>> Otherwise, when device_add() fails in internal and calls kobject_put()
>>> in error handling path, it would report warning because the device
>>> never be registered as chardev and there is no release function for it.
>>>
>>> Fixes: 8ebaa3ff1e71 ("iio: core: register chardev only if needed")
>>> Signed-off-by: Zeng Heng <zengheng4@huawei.com>
>> Interesting corner case. The cdev_device_add() call is fine with
>> !dev->devt which is what this code was taking advantage of. The 
>> exception
>> as you have highlighted is the error path of device_add().
>>
>> So I think it should also cope with unwinding if device_add() fails
>> and not be calling cdev_del()  Note that cdev_device_del() has the
>> appropriate guards to be safe whether or not (dev->devt) is true.
>>
>> Perhaps change cdev_device_add() to have
>>
>>     rc = device_add(dev);
>>     if (rc && dev->devt)
>>         cdev_del(cdev);
>>
>>     return rc;
>
yes, I agree with your opinion about cdev_device_add(), which would be 
more flexible

for callers.

And I *just find* there is existing patch trying to fix this out.

https://lore.kernel.org/lkml/Y1fmgCS7fuf%2FLQBc@kroah.com/

So just pass the patch.


Best Regards,

Zeng Heng

>>> ---
>>>   drivers/iio/industrialio-core.c | 11 +++++++++--
>>>   1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/iio/industrialio-core.c 
>>> b/drivers/iio/industrialio-core.c
>>> index 151ff3993354..f4f48bda07f7 100644
>>> --- a/drivers/iio/industrialio-core.c
>>> +++ b/drivers/iio/industrialio-core.c
>>> @@ -1982,7 +1982,11 @@ int __iio_device_register(struct iio_dev 
>>> *indio_dev, struct module *this_mod)
>>>       /* assign device groups now; they should be all registered now */
>>>       indio_dev->dev.groups = iio_dev_opaque->groups;
>>>   -    ret = cdev_device_add(&iio_dev_opaque->chrdev, &indio_dev->dev);
>>> +    if (iio_dev_opaque->attached_buffers_cnt || 
>>> iio_dev_opaque->event_interface)
>>> +        ret = cdev_device_add(&iio_dev_opaque->chrdev, 
>>> &indio_dev->dev);
>>> +    else
>>> +        ret = device_add(&indio_dev->dev);
>>> +
>>>       if (ret < 0)
>>>           goto error_unreg_eventset;
>>>   @@ -2008,7 +2012,10 @@ void iio_device_unregister(struct iio_dev 
>>> *indio_dev)
>>>   {
>>>       struct iio_dev_opaque *iio_dev_opaque = 
>>> to_iio_dev_opaque(indio_dev);
>>>   -    cdev_device_del(&iio_dev_opaque->chrdev, &indio_dev->dev);
>>> +    if (iio_dev_opaque->chrdev.kobj.state_initialized)
>>> +        cdev_device_del(&iio_dev_opaque->chrdev, &indio_dev->dev);
>>> +    else
>>> +        device_del(&indio_dev->dev);
>>>         mutex_lock(&iio_dev_opaque->info_exist_lock);