Not restarting for due to signals.

This builds on patches 1-15 of my previous patch posting. As those are non-controversial I am not posting them again.

I took longer than I had hoped to get this set together because a kernel testing robot noticed some random corruption with the way I had been adding to the list. I finally tracked it down to failing to remove the sigset from the list during fork_idle. So I have made that logic simpler and use hlist_del_init which will only remove an item from a list if it was placed on the list in the first place.

I took Oleg's suggesting and moved calculate_sigpending into schedule_tail where recalc_sigpending an be used directly. Then in calculate_sigpending I just unconditionally set TIF_SIGPENDING and allow recalc_sigpending to clear TIF_SIGPENDING if we don't need it.

I also now handle the stop/continue signal magic where we only let one of stop signals and SIGCONT be pending at a time. Looking at it from first principles dropping one of SIGTSTP SIGTTIN SIGTTOU or SIGCONT before calling it's handler feels wrong. I checked and it is our historical behavior, so I won't even thinking of introducing different behavior at this point.

Eric W. Biederman (6):
fork: Move and describe why the code examines PIDNS_ADDING
fork: Unconditionally exit if a fatal signal is pending
signal: Add calculate_sigpending()
fork: Skip setting TIF_SIGPENDING in ptrace_init_task
fork: Have new threads join on-going signal group stops
signal: Don't restart fork when signals come in.

 include/linux/ptrace.h       |  2 --
 include/linux/sched/signal.h | 11 ++++++++++
 init/init_task.c             |  1 +
 kernel/fork.c                | 49 ++++++++++++++++++++++++++++++--------------
 kernel/sched/core.c          |  2 ++
 kernel/signal.c              | 43 ++++++++++++++++++++++++++++++++++++++
 6 files changed, 91 insertions(+), 17 deletions(-)

Eric