lsmod shows -2 in the “Used by” column

In your kernel configuration (make config, make menuconfig etc.) you need to enable CONFIG_MODULE_UNLOAD:

• When CONFIG_MODULE_UNLOAD is set, the kernel counts references, as you may only unload a module if there are no references to it.

• When CONFIG_MODULE_UNLOAD is not set, then the kernel has no need to count how many references there are to a module, and it always returns -2 as a marker value.

This answer originally came from Gentoo Forums.