Deleting a sysfs entry - linux-kernel

Question

i was trying to learn sysfs and was trying to write a simple sysfs directory. The code is as below
static struct kobject *example_kobject;
static int __init mymodule_init (void)
{
pr_debug("Module initialized successfully \n");
example_kobject = kobject_create_and_add("kobject_example",
kernel_kobj);
if(!example_kobject)
return -ENOMEM;
return 0;
}
static void __exit mymodule_exit (void)
{
pr_debug ("Module un initialized successfully \n");
// kobject_put(example_kobject); <-- Forgot to delete
}
module_init(mymodule_init);
module_exit(mymodule_exit);
As shown in the mymodule_exit, i had by mistake forgot to uncomment the code and then inserted and rmmod the module.
Now when i try to insert the module again, the initialization is failing as the entry is already present.
I know, it does not make sense to allow userspace to remove the entry that the kernel made. But, i was still wondering if there is any other way to remove the particular /sys/kernel/kobject_example entry other than rebooting the box.

Answer 1

Firstly, merely doing kobject_put() is not good enough, you must use kobject_del() instead. kobject_put() does not do complete clean-up. In your case, since kobject_example is a file (not dir), mere 'put' will still leave the entry in the parent dir (kset).
If you must, there is a away to remove such an entry without reboot, and that is by writing another module to do that. Here is what the module should be doing:
/* Find the kobj from the path and parent kset */
kobj = kset_find_obj(kernel_kobj->kset, "kobject_example");
...
/* check kobj is not null etc. */
...
/* Remove the sysfs entry */
kobject_del(kobj);
This will delete the sysfs entry. Reboot is easy, but this is nifty when your system does not have an option to go out of service.