reduce tasklist_lock hold time on exit and do some pid cleanup

The clone side contends against exit side in a way which avoidably
exacerbates the problem by the latter waiting on locks held by the
former while holding the tasklist_lock.

Whacking this for both add_device_randomness and pids allocation gives
me a 15% speed up for thread creation/destruction in a 24-core vm.

The random patch is worth about 4%.

nothing blew up with lockdep, lightly tested so far

Bench (plop into will-it-scale):
$ cat tests/threadspawn1.c

char *testcase_description = "Thread creation and teardown";

static void *worker(void *arg)
{
	return (NULL);
}

void testcase(unsigned long long *iterations, unsigned long nr)
{
	pthread_t thread;
	int error;

	while (1) {
		error = pthread_create(&thread, NULL, worker, NULL);
		assert(error == 0);
		error = pthread_join(thread, NULL);
		assert(error == 0);
		(*iterations)++;
	}
}

v5:
- whack scripts/selinux/genheaders/genheaders which accidentally got in
- rebased on next-20250205

v4:
- justify moving get_pid in the commit message with a one-liner
- drop the tty unref patch -- it is completely optional and Oleg has his
  own variant
- add the ACK by Oleg

v3:
- keep procfs flush where it was, instead hoist get_pid outside of the
  lock
- make detach_pid et al accept an array argument of pids to populate
- sprinkle asserts
- drop irq trips around pidmap_lock
- move tty unref outside of tasklist_lock

Mateusz Guzik (5):
  exit: perform add_device_randomness() without tasklist_lock
  exit: hoist get_pid() in release_task() outside of tasklist_lock
  pid: sprinkle tasklist_lock asserts
  pid: perform free_pid() calls outside of tasklist_lock
  pid: drop irq disablement around pidmap_lock

 include/linux/pid.h |  7 ++--
 kernel/exit.c       | 36 +++++++++++++-------
 kernel/pid.c        | 82 +++++++++++++++++++++++++--------------------
 kernel/sys.c        | 14 +++++---
 4 files changed, 82 insertions(+), 57 deletions(-)

-- 
2.43.0