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
