rcu/tasks: Handle new PF_IDLE semantics

rcu/tasks: Handle new PF_IDLE semantics

[ Upstream commit 9715ed501b585d47444865071674c961c0cc0020 ]

The commit:

	cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")

has changed the semantics of what is to be considered an idle task in
such a way that CPU boot code preceding the actual idle loop is excluded
from it.

This has however introduced new potential RCU-tasks stalls when either:

1) Grace period is started before init/0 had a chance to set PF_IDLE,
   keeping it stuck in the holdout list until idle ever schedules.

2) Grace period is started when some possible CPUs have never been
   online, keeping their idle tasks stuck in the holdout list until the
   CPU ever boots up.

3) Similar to 1) but with secondary CPUs: Grace period is started
   concurrently with secondary CPU booting, putting its idle task in
   the holdout list because PF_IDLE isn't yet observed on it. It stays
   then stuck in the holdout list until that CPU ever schedules. The
   effect is mitigated here by the hotplug AP thread that must run to
   bring the CPU up.

Fix this with handling the new semantics of PF_IDLE, keeping in mind
that it may or may not be set on an idle task. Take advantage of that to
strengthen the coverage of an RCU-tasks quiescent state within an idle
task, excluding the CPU boot code from it. Only the code running within
the idle loop is now a quiescent state, along with offline CPUs.

Fixes: cff9b2332ab7 ("kernel/sched: Modify initial boot task idle setup")
Suggested-by: Joel Fernandes <joel@joelfernandes.org>
Suggested-by: "Paul E. McKenney" <paulmck@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>