On Wed, Aug 28, 2019 at 11:27:39AM +0200, Sebastian Andrzej Siewior wrote: > 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(). This one, right? WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0); Another approach would be to change that WARN_ON_ONCE(). This fix might be too extreme, as it would suppress other issues: WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && !preempt && t->rcu_read_lock_nesting > 0); But maybe what is happening under the covers is that preempt is being set when sleeping on a spinlock. Is that the case? Thanx, Paul
