[PATCH tip/core/rcu 0/6] Remove scheduler restriction, update for v5.8

[Paul E. McKenney]

Hello!

This series removes the long-standing restriction preventing holding
of scheduler locks across rcu_read_unlock() unless interrupts have
been disabled across the entire RCU read-side critical section.

For example, with this series, the following code is now perfectly legal,
and is in fact tested by rcutorture:

	// Preemption is enabled.
	rcu_read_lock();
	do_something();
	raw_spin_lock_irqsave(&current->pi_lock, flags);
	rcu_read_unlock();
	raw_spin_unlock_irqrestore(&current->pi_lock, flags);

This change was enabled by the earlier changes that turn on
scheduler-clock interrupts for nohz_full CPUs that take too long to
report a quiescent state during a given grace period.  (Yes, nohz_full
CPUs executing in userspace still have their quiescent states implicitly
reported by the RCU grace-period kthread!)

And here is the series:

1.	Add KCSAN stubs to update.c.

2.	Make rcu_read_unlock_special() safe for rq/pi locks.

3.	Don't set nesting depth negative in rcu_preempt_deferred_qs(),
	courtesy of Lai Jiangshan.

4.	Remove unused ->rcu_read_unlock_special.b.deferred_qs field,
	courtesy of Lai Jiangshan.

5.	Don't use negative nesting depth in __rcu_read_unlock(),
	courtesy of Lai Jiangshan.

6.	Add test of holding scheduler locks across rcu_read_unlock().

							Thanx, Paul

------------------------------------------------------------------------

 include/linux/sched.h    |    2 +-
 kernel/rcu/rcutorture.c  |   10 +++++++++-
 kernel/rcu/tree_exp.h    |   31 +++++--------------------------
 kernel/rcu/tree_plugin.h |   45 +++++++++++++++------------------------------
 kernel/rcu/update.c      |   13 +++++++++++++
 5 files changed, 43 insertions(+), 58 deletions(-)