linux-next: add utrace tree

[Oleg Nesterov]

On 01/26, Andi Kleen wrote:
>
> But when I did that i couldn't come up with a good scenario
> where multiple debuggers actually make sense. In a sense
> being a debugger is really a very "intimate" thing for process. Do you
> really want to have multiple of them messing with each other?
>
> If yes how would they know what to touch and what not?

Yes, multiple debuggers can confuse each other if they change
the state of debuggee simultaneously. The user should do this ;)

> Can you think of any scenario where multiple debuggers
> on a process make sense?

Simple example. Try to debug/strace strace ot gdb itself. Not trivial,
you can't attach to strace's tracees. Recently I spent 2 days trying to
understand why strace -f hangs. I was able to attach to strace, but
I wasn't able to see what its tracees do.

And, it was not possible to even trace strace until it hangs, with
ptrace the tracee (strace) must stop to report the event and this
shadowed the race.

Oleg.