On 2019-08-27 08:53:06 [-0700], Paul E. McKenney wrote:

quoted

quoted

On the other hand, within a PREEMPT=n kernel, the call to schedule()
would split even an rcu_read_lock() critical section.  Which is why I
asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops
in !PREEMPT_RT_BASE kernels.  We would after all want the usual lockdep
complaints in that case.

sleeping_lock_inc() +dec() is only RT specific. It is part of RT's
spin_lock() implementation and used by RCU (rcu_note_context_switch())
to not complain if invoked within a critical section.

Then this is being called when we have something like this, correct?

	DEFINE_SPINLOCK(mylock); // As opposed to DEFINE_RAW_SPINLOCK().

	...

	rcu_read_lock();
	do_something();
	spin_lock(&mylock); // Can block in -rt, thus needs sleeping_lock_inc()
	...
	rcu_read_unlock();

Without sleeping_lock_inc(), lockdep would complain about a voluntary
schedule within an RCU read-side critical section.  But in -rt, voluntary
schedules due to sleeping on a "spinlock" are OK.

Am I understanding this correctly?

Everything perfect except that it is not lockdep complaining but the
WARN_ON_ONCE() in rcu_note_context_switch().

							Thanx, Paul

Sebastian