Re: A potential broken at platform driver?

[Richard Gong <richard.gong@linux.intel.com>]

Hi Greg,

On 6/4/19 9:28 AM, Greg KH wrote:
> On Tue, Jun 04, 2019 at 12:33:03PM +0200, Romain Izard wrote:
>> On Mon, Jun 03, 2019 at 08:02:55PM +0200, Greg KH wrote:
>>>> @@ -394,7 +432,7 @@ static struct platform_driver stratix10_rsu_driver = {
>>>>   	.remove = stratix10_rsu_remove,
>>>>   	.driver = {
>>>>   		.name = "stratix10-rsu",
>>>> -		.groups = rsu_groups,
>>>> +//		.groups = rsu_groups,
>>>
>>> Are you sure this is the correct pointer?  I think that might be
>>> pointing to the driver's attributes, not the device's attributes.
>>>
>>> If platform drivers do not have a way to register groups properly, then
>>> that really needs to be fixed, as trying to register it by yourself as
>>> you are doing, is ripe for racing with userspace.
>>   
>> This is a very common issue with platform drivers, and it seems to me that
>> it is not possible to add device attributes when binding a device to a
>> driver without entering the race condition.
>>
>> My understanding is the following one:
>>
>> The root cause is that the device has already been created and reported
>> to the userspace with a KOBJ_ADD uevent before the device and the driver
>> are bound together. On receiving this event, userspace will react, and
>> it will try to read the device's attributes. In parallel the kernel will
>> try to find a matching driver. If a driver is found, the kernel will
>> call the probe function from the driver with the device as a parameter,
>> and if successful a KOBJ_BIND uevent will be sent to userspace, but this
>> is a recent addition.
>>
>> Unfortunately, not all created devices will be bound to a driver, and the
>> existing udev code relies on KOBJ_ADD uevents rather than KOBJ_BIND uevents.
>> If new per-device attributes have been added to the device during the
>> binding stage userspace may or may not see them, depending on when userspace
>> tries to read the device's attributes.
>>
>> I have this possible workaround, but I do not know if it is a good solution:
>>
>> When binding the device and the driver together, create a new device as a
>> child to the current device, and fill its "groups" member to point to the
>> per-device attributes' group. As the device will be created with all the
>> attributes, it will not be affected by the race issues. The functions
>> handling the attributes will need to be modified to use the parents of their
>> "device" parameter, instead of the device itself. Additionnaly, the sysfs
>> location of the attributes will be different, as the child device will show
>> up in the sysfs path. But for a newly introduced device this will not be
>> a problem.
>>
>> Is this a good compromise ?
> 
> Not really.  You just want the attributes on the platform device itself.
> 
> Given the horrible hack that platform devices are today, what's one more
> hack!
> 
> Here's a patch below of what should probably be done here.  Richard, can
> you change your code to use the new dev_groups pointer in the struct
> platform_driver and this patch and let me know if that works or not?
> 
> Note, I've only compiled this code, not tested it...
>

Your patch works.

Many thanks for your help!

Regards,
Richard

> thanks,
> 
> greg k-h
> 
> diff --git a/drivers/base/platform.c b/drivers/base/platform.c
> index 4d1729853d1a..3dd4b73a9b30 100644
> --- a/drivers/base/platform.c
> +++ b/drivers/base/platform.c
> @@ -598,6 +598,7 @@ struct platform_device *platform_device_register_full(
>   }
>   EXPORT_SYMBOL_GPL(platform_device_register_full);
>   
> +static int platform_drv_remove(struct device *_dev);
>   static int platform_drv_probe(struct device *_dev)
>   {
>   	struct platform_driver *drv = to_platform_driver(_dev->driver);
> @@ -614,8 +615,18 @@ static int platform_drv_probe(struct device *_dev)
>   
>   	if (drv->probe) {
>   		ret = drv->probe(dev);
> -		if (ret)
> +		if (ret) {
>   			dev_pm_domain_detach(_dev, true);
> +			goto out;
> +		}
> +	}
> +	if (drv->dev_groups) {
> +		ret = device_add_groups(_dev, drv->dev_groups);
> +		if (ret) {
> +			platform_drv_remove(_dev);
> +			return ret;
> +		}
> +		kobject_uevent(&_dev->kobj, KOBJ_CHANGE);
>   	}
>   
>   out:
> @@ -640,6 +651,8 @@ static int platform_drv_remove(struct device *_dev)
>   
>   	if (drv->remove)
>   		ret = drv->remove(dev);
> +	if (drv->dev_groups)
> +		device_remove_groups(_dev, drv->dev_groups);
>   	dev_pm_domain_detach(_dev, true);
>   
>   	return ret;
> diff --git a/include/linux/platform_device.h b/include/linux/platform_device.h
> index cc464850b71e..027f1e1d7af8 100644
> --- a/include/linux/platform_device.h
> +++ b/include/linux/platform_device.h
> @@ -190,6 +190,7 @@ struct platform_driver {
>   	int (*resume)(struct platform_device *);
>   	struct device_driver driver;
>   	const struct platform_device_id *id_table;
> +	const struct attribute_group **dev_groups;
>   	bool prevent_deferred_probe;
>   };
>   
>