Comment 58 for bug 1314556

Tested with mainline kernels

https://wiki.ubuntu.com/Kernel/MainlineBuilds
http://kernel.ubuntu.com/~kernel-ppa/mainline/

On the most recent kernel v4.2-rc8-unstable problem persists (lsusb hangs, MTP/ADB do NOT work)
http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.2-rc8-unstable/

On kernel v3.17.8-vivid no problem (lsusb is instant, MTP/ADB DO WORK)
http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.17.8-vivid/

On kernel v3.18-rc1-utopic the problem appers for the 1st time (lsusb hangs, MTP/ADB do NOT work)
http://kernel.ubuntu.com/~kernel-ppa/mainline/v3.18-rc1-utopic/

to install, download deb and run
sudo dpkg -i linux-image-3.17.8-031708-generic_3.17.8-031708.201501081837_amd64.deb
the new kernel item will go to grub menu

So, the regression was added to the kernel during v3.17.8-vivid -> v3.18-rc1-utopi transition. Someone who is competent could try to make a diff or something.

Aslo, as a side note, it seems that on 3.16/3.17 kernels 2 other annoying problems from the default Ubuntu 14.04 installation with kernel 3.19 are gone:
- mouse cursor did disappear for some time almost after desktop load, then appeared with no visible reason
- keyboard switcher could stop working at some random moment for some time and randomly returned back to life

This may have the same source in hanging usb subsystem which may block mouse/keyboard calls in some way for some time and then return.