Still working on this. Here is some dmesg spew:

[  514.245846] foobar: module verification failed: signature and/or required key missing - tainting kernel
[  514.245937] kobject: 'foobar' (f7f060c8): kobject_add_internal: parent: 'module', set: 'module'
[  514.245951] kobject: 'holders' (f5ff3d40): kobject_add_internal: parent: 'foobar', set: '<NULL>'
[  514.245981] kobject: 'notes' (f2d25f80): kobject_add_internal: parent: 'foobar', set: '<NULL>'
[  514.245987] kobject: 'foobar' (f7f060c8): kobject_uevent_env
[  514.245998] kobject: 'foobar' (f7f060c8): fill_kobj_path: path = '/module/foobar'

So it looks like kernel validation is failing. I have printk's in my init fxn that are never turning up in /var/log/messages, until, weirdly, AFTER I remove the device:

<insmod device>
Dec 30 09:43:03 localhost kernel: [  514.245846] foobar: module verification failed: signature and/or required key missing - tainting kernel
Dec 30 09:43:16 localhost fprintd[1085]: ** Message: No devices in use, exit

<rmmod device>
Dec 30 09:45:53 localhost kernel: [  514.249323] foobar: got device number 248, minor is 0   <<<<----THIS IS IN init() fxn
Dec 30 09:45:53 localhost kernel: [  684.102912] unregister_chrdev(248) called for foobar<7>[  684.102927] kobject: '(null)' (f7f06220): kobject_cleanup, parent   (null)

<insmod>
insmod: ERROR: could not insert module ./foobar.ko: Device or resource busy








On Fri, Dec 27, 2013 at 9:13 PM, <Valdis.Kletnieks@vt.edu> wrote:
On Fri, 27 Dec 2013 19:33:50 -0800, Eric Fowler said:

> I suspect I am doing something wrong in the code with
> register/unregister_chrdev(), but I have been over that code a million
> times. It looks fine.
>
> Now:
> insmod the device, OK
> rmmod the device, OK
> Check /proc/devices , device # is present
> insmod the device again, fails with ERROR: could not insert module
> ./foobar.ko: Device or resource busy

It does smell like an unregister issue.  You may want to try adding
printk() calls to print out the return code from register and unregister.
I'm willing to bet that (a) the unegister is failing because somebody
still has a reference on the device, and (b) the second register call fails
because the device already exists, causing your module_init() to bail out.

The fun is that you may not have taken a reference on the device directly
yourself - you may have called some other get_foo() that ends up taking an
implicit reference under the covers, causing issues when you fail to call
put_foo() at the right place...



--
cc:NSA