Re: Simplify restart logic and fix races

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Oren Laadan (orenl-RdfvBDnrOixBDgjK7y7TUQ@public.gmane.org):
> This series simplifies adn tightens the restart logic and solves a
> couple of races that annoyed Serge recently:
> 
> *  c/r: simplify logic of tracking restarting tasks
> *  c/r: let entire thread group in sys_restart before restoring a thread
> *  c/r: introduce walk_task_subtree() to iterate through descendants
> *  c/r: fix restart failure due to a race with ghost/dead tasks
> *  c/r: defer restore of blocked signals mask during restart
> 
> Applies on ckpt-v18, works well on my tests.
> 
> Oren.

I want to review these in detail tomorrow, so I'll send acks
then, but definately

Tested-by: Serge Hallyn <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

This passes all of the tests in git://git.sr71.net/~hallyn/cr_tests.git
repeatedly.

thanks,
-serge