On Sun, Aug 18, 2019 at 09:41:43PM -0400, Joel Fernandes wrote: > On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote: [snip] > > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > > > which implies raising softirq will not do any wake ups." This mention > > > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > > > > > Yes, I should delete this #2 from the changelog since it is not very relevant > > > (I feel now). My point with #2 was that even if were to raise a softirq > > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > > > path anyway since in_irq() implies in_interrupt(). > > > > Please! Could you also add a first-principles explanation of why > > the added condition is immune from scheduler deadlocks? > > Sure I can add an example in the change log, however I was thinking of this > example which you mentioned: > https://lore.kernel.org/lkml/20190627173831.GW26519@xxxxxxxxxxxxx/ > > previous_reader() > { > rcu_read_lock(); > do_something(); /* Preemption happened here. */ > local_irq_disable(); /* Cannot be the scheduler! */ > do_something_else(); > rcu_read_unlock(); /* Must defer QS, task still queued. */ > do_some_other_thing(); > local_irq_enable(); > } > > current_reader() /* QS from previous_reader() is still deferred. */ > { > local_irq_disable(); /* Might be the scheduler. */ > do_whatever(); > rcu_read_lock(); > do_whatever_else(); > rcu_read_unlock(); /* Must still defer reporting QS. */ > do_whatever_comes_to_mind(); > local_irq_enable(); > } > > One modification of the example could be, previous_reader() could also do: > previous_reader() > { > rcu_read_lock(); > do_something_that_takes_really_long(); /* causes need_qs in > the unlock_special_union to be set */ > local_irq_disable(); /* Cannot be the scheduler! */ > do_something_else(); > rcu_read_unlock(); /* Must defer QS, task still queued. */ > do_some_other_thing(); > local_irq_enable(); > } The point you were making in that thread being, current_reader() -> rcu_read_unlock() -> rcu_read_unlock_special() would not do any wakeups because previous_reader() sets the deferred_qs bit. Anyway, I will add all of this into the changelog. thanks, - Joel
