TASK_KILLABLE

If it's in KILLABLE state, it *does* know how to deal with signals within 
the kernel, and its in-kernel state can be cleanly unwound. The reason 
that this doesn't propagate up to userspace as an EINTR is simply that 
there is in effect a Unix guarantee that filesystem operations cannot be 
interrupted, and the vast majority of userspace code relies on this 
guarantee and will malfunction if it starts getting EINTRs from tasks. 
(This is what the old 'intr' option did, and boy were the results messy.)

That's why it only responds to SIGKILL: because SIGKILL, by definition, 
doesn't get propagated to userspace, because the process's userspace 
component is killed by the SIGKILL.