Re: [QUESTION/sysfs] strange refcounting

[Greg KH <greg@kroah.com>]

On Fri, Feb 03, 2006 at 05:16:22PM +0300, Artem B. Bityutskiy wrote:
> Hello folks,
> 
> I'm writing a simple device driver and want to expose some of its 
> attributes to userspace via sysfs.
> 
> As usually, I have main device description structure "struct 
> mydev_info". I've embedded a struct device object there. What I do is:
> 
> struct mydev_info mydev
> {
> 	struct device *dev;

First off, this should not be a pointer, but rather:
	struct device dev;

That properly embedds the struct device into your object.

> 	... bla bla bla ...
> } mydev;
> 
> 
> mydev->dev=kzalloc(sizeof(struct device), GFP_KERNEL);
> mydev->dev->bus_id = "mydev";
> mydev->dev->release = mydev_release;
> err = device_register(&mydev->dev);

What type of bus does this device live on?  You should not be calling
device_register() on your own directly.  Either use a bus, and be a
device of it, or use the platform_device() interface.

> Then, I see /sys/devices/mydev/ in sysfs. I open pre-defined 
> /sys/devices/mydev/power/state in userspace and don't close it.
> 
> Then I run lsmod, and see zero refcount to my module. Well, I run rmmod 
> mymod, module is unloaded.

Yup.

> Then I close /sys/devices/mydev/power/state, and enjoy segfault.

What is the backtrace?

> I thought sysfs subsystem have to increase module refcount when one 
> opens its sysfs files. Well, there is a release function, but it is also 
> unloaded with the module.

Again, register with a bus or use the platform_device() interface, and
this should work properly.

> May be there is a problem because of I have mydev->dev->parent == NULL, 
> mydev->dev->bus == NULL, mydev->dev->driver == NULL? But I really don't 
> have any bus, any parent and I don't want to introduce struct 
> device_driver ...

Yes, you kind of need all of that :)

Make the above changes and let us know if that helps things.

thanks,

greg k-h
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