Merge patch series "reduce tasklist_lock hold time on exit and do some pid cleanup"

Mateusz Guzik <mjguzik@gmail.com> says:

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%.

The new bottleneck is pidmap_lock itself, with the biggest problem being
the allocation itself taking the lock *twice*.

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)++;
	}
}

* patches from https://lore.kernel.org/r/20250206164415.450051-1-mjguzik@gmail.com:
  pid: drop irq disablement around pidmap_lock
  pid: perform free_pid() calls outside of tasklist_lock
  pid: sprinkle tasklist_lock asserts
  exit: hoist get_pid() in release_task() outside of tasklist_lock
  exit: perform add_device_randomness() without tasklist_lock

Link: https://lore.kernel.org/r/20250206164415.450051-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>