Linux block device module is hanging when unloading with rmmod (del_gendisk) - c

Question

I'm writing a little example for block device drivers in Linux. This example is not complete and I take progress step by step. I registered a block device with blkdev_register and allocated the gendisk structure with alloc_disk. All works fine, when inserting the module. It shows up in /proc/devices. But if I want to unload it with rmmod, it hangs.
I figured out, that in the module unload function, the call to del_gendisk caused the hanging. I know that the gendisk structure has an embedded kobject, which takes care of reference counting. This mechanism hinders you to unload the module, while it's being used. But since I don't call add_disk, there should be no reference's to that structure.
#include <linux/module.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/genhd.h>
#include <linux/blkdev.h>
#define BLKDEVNAME "blkdev_test"
#define MINORS 16
struct block_device_operations bdops = {
.owner = THIS_MODULE
};
struct blkdev {
int major;
struct gendisk *disk;
struct request_queue *queue;
} dev;
static int __init blkdev_init(void)
{
dev.major = register_blkdev( 0, BLKDEVNAME );
if( dev.major < 0 )
return -EIO;
dev.disk = alloc_disk(MINORS);
if( dev.disk == NULL )
goto DISK_ERR;
dev.disk->major = dev.major;
dev.disk->first_minor = 0;
snprintf(dev.disk->disk_name, DISK_NAME_LEN, "bd0" );
dev.disk->fops = &bdops;
// dev.disk->queue = dev.queue;
// add_disk( dev.disk );
return 0;
DISK_ERR:
unregister_blkdev( dev.major, BLKDEVNAME );
return -EIO;
}
static void __exit blkdev_exit(void)
{
del_gendisk(dev.disk);
unregister_blkdev( dev.major, BLKDEVNAME );
}
module_init(blkdev_init);
module_exit(blkdev_exit);
MODULE_LICENSE("GPL");
If I issue the command sudo rmmod mod.ko then the command gets killed by the system.

Answer 1

Quoting :
Manipulating gendisks Once you have your gendisk structure set up, you
have to add it to the list of active disks; that is done with:
void add_disk(struct gendisk *disk);
After this call, your device is active. There are a few things worth
keeping in mind about add_disk():
add_disk() can create I/O to the device (to read partition tables and such). You should not call add_disk() until your driver is
sufficiently initialized to handle requests.
If you are calling add_disk() in your driver initialization routine, you should not fail the initialization process after the
first call.
The call to add_disk() increments the disk's reference count; if the disk structure is ever to be released, the driver is responsible
for decrementing that count (with put_disk()).
Should you need to remove a disk from the system, that is accomplished
with:
void del_gendisk(struct gendisk *disk);
This function cleans up all of the information associated with the
given disk, and generally removes it from the system. After a call to
del_gendisk(), no more operations will be sent to the given device.
Your driver's reference to the gendisk object remains, though; you
must explicitly release it with:
void put_disk(struct gendisk *disk);
That call will cause the gendisk structure to be freed, as long as no
other part of the kernel retains a reference to it.