[v3.13][v3.14][Regression] kthread: make kthread_create() killable

[One Thousand Gnomes]

> Root cause time: it's wrong for the oom-killer to use SIGKILL.  In fact

It has to use SIGKILL anything else might be caught and grow the user
stack a page..

> it's basically always wrong to send signals from in-kernel.  Signals
> are a userspace IPC mechanism and using them in-kernel a) makes it hard
> (or impossible) to distinguish them from userspace-originated signals

Actually signals are a kernel messaging system someone repurposed for IPC.

> and b) permits userspace to produce surprising results in the kernel,
> which I suspect is what we're seeing here.

There is enough information for kernel side code to decide whether a
signal came from kernel or userspace. Then again - it's not clear that it
should ever have to.

Alan