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    SubjectRe: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep
    On 2019-08-27 08:53:06 [-0700], Paul E. McKenney wrote:
    > > > 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

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