Upon reevaluation we think it is indeed safe to permit the race between
a ptrace call and the traced thread waking up, as long as it will never
get back to user mode.  This patch makes SIGKILL wake up threads in
TASK_TRACED.  That alone resolves most of the deadlock issues that
became possible with the introduction of TASK_TRACED, getting us back to
the killing behavior of 2.6.8 and before. 

This patch also further cleans up ptrace detaching, so that threads are
left in TASK_STOPPED only if a job control stop is actually in effect,
and otherwise resume.  This removes the past nuisances requiring a
SIGCONT to resume a thread even when it had a pending SIGKILL. 
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>