backtrace not working on mainline x86_64

[Frank Ch. Eigler]

hien wrote:

> [...]  The permanent fix for this would be implement the
> dump_stack_to_buffer() function and push it to the main line [...]

Let me raise the perfunctory caution against relying on heavyweight
routines in the kernel for tasks also performable by the runtime.
("heavyweight" here means using resources like locks in such a way
that makes them unsafe for calling from inconvenient probe contexts.)

- FChE