proc: Ensure we see the exit of each process tid exactly

proc: Ensure we see the exit of each process tid exactly

In the work to remove proc_mnt I noticed that we were calling
proc_flush_task now proc_flush_pid possibly multiple times for the same
pid because of how de_thread works.

This is a bare minimal patchset to sort out de_thread, by introducing
exchange_tids and the helper of exchange_tids hlists_swap_heads_rcu.

The actual call of exchange_tids should be slowpath so I have
prioritized readability over getting every last drop of performance.

I have also read through a bunch of the code to see if I could find
anything that would be affected by this change.  Users of
has_group_leader_pid were a good canidates.  But I also looked at other
cases that might have a pid->task->pid transition.  I ignored other
sources of races with de_thread and exec as those are preexisting.

I found a close call with send_signals user of task_active_pid_ns, but
all pids of a thread group are guaranteeds to be in the same pid
namespace so there is not a problem.

I found a few pieces of debugging code that do:

	task = pid_task(pid, PIDTYPE_PID);
	if (task) {
		printk("%u\n", task->pid);
	}

But I can't see how we care if it happens at the wrong moment that
task->pid might not match pid_nr(pid);

Similarly because the code in posix-cpu-timers goes pid->task->pid it
feels like there should be a problem.  But as the code that works with
PIDTYPE_PID is only available within the thread group, and as de_thread
kills all of the other threads before it makes any changes of this
kind the race can not happen.

In short I don't think this change will introduce any regressions.

Eric W. Biederman (2):
      rculist: Add hlists_swap_heads_rcu
      proc: Ensure we see the exit of each process tid exactly once

 fs/exec.c               |  5 +----
 include/linux/pid.h     |  1 +
 include/linux/rculist.h | 21 +++++++++++++++++++++
 kernel/pid.c            | 19 +++++++++++++++++++
 4 files changed, 42 insertions(+), 4 deletions(-)

Link: https://lore.kernel.org/lkml/87sggnajpv.fsf_-_@x220.int.ebiederm.org/
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>