On Sun, Aug 18, 2019 at 07:29:27PM -0700, Paul E. McKenney wrote: > On Sun, Aug 18, 2019 at 09:46:23PM -0400, Joel Fernandes wrote: > > 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. > > Examples are good, but what makes it so that there are no examples of > its being unsafe? > > And a few questions along the way, some quick quiz, some more serious. > Would it be safe if it checked in_interrupt() instead of in_irq()? > If not, should the in_interrupt() in the "if" condition preceding the > added "else if" be changed to in_irq()? Would it make sense to add an > "|| !irqs_were_disabled" do your new "else if" condition? Would the > body of the "else if" actually be executed in current mainline? > > In an attempt to be at least a little constructive, I am doing some > testing of this patch overnight, along with a WARN_ON_ONCE() to see if > that invoke_rcu_core() is ever reached. And that WARN_ON_ONCE() never triggered in two-hour rcutorture runs of TREE01, TREE02, TREE03, and TREE09. (These are the TREE variants in CFLIST that have CONFIG_PREEMPT=y.) This of course raises other questions. But first, do you see that code executing in your testing? Thanx, Paul
