On Fri, Oct 14, 2022 at 04:40:19PM +0200, Frederic Weisbecker wrote: > On Fri, Oct 14, 2022 at 07:21:27AM -0700, Paul E. McKenney wrote: > > On Tue, Oct 11, 2022 at 06:01:30PM +0000, Joel Fernandes (Google) wrote: > > > From: Frederic Weisbecker <frederic@xxxxxxxxxx> > > > > > > Upon entraining a callback to a NOCB CPU, no further wake up is > > > issued on the corresponding nocb_gp kthread. As a result, the callback > > > and all the subsequent ones on that CPU may be ignored, at least until > > > an RCU_NOCB_WAKE_FORCE timer is ever armed or another NOCB CPU belonging > > > to the same group enqueues a callback on an empty queue. > > > > > > Here is a possible bad scenario: > > > > > > 1) CPU 0 is NOCB unlike all other CPUs. > > > 2) CPU 0 queues a callback > > > > Call it CB1. > > > > > 2) The grace period related to that callback elapses > > > 3) The callback is moved to the done list (but is not invoked yet), > > > there are no more pending callbacks for CPU 0 > > > > So CB1 is on ->cblist waiting to be invoked, correct? > > > > > 4) CPU 1 calls rcu_barrier() and sends an IPI to CPU 0 > > > 5) CPU 0 entrains the callback but doesn't wake up nocb_gp > > > > And CB1 must still be there because otherwise the IPI handler would not > > have entrained the callback, correct? If so, we have both CB1 and the > > rcu_barrier() callback (call it CB2) in ->cblist, but on the done list. > > > > > 6) CPU 1 blocks forever, unless CPU 0 ever queues enough further > > > callbacks to arm an RCU_NOCB_WAKE_FORCE timer. > > > > Except that -something- must have already been prepared to wake up in > > order to invoke CB1. And that something would invoke CB2 along with CB1, > > given that they are both on the done list. If there is no such wakeup > > already, then the hang could occur with just CB1, without the help of CB2. > > Heh good point. I was confused with CB1 on RCU_DONE_TAIL and the possibility > for CB2 to be entrained on RCU_WAIT_TAIL. But that's indeed not supposed to > happen. Ok so this patch indeed doesn't make sense outside lazy. Whew!!! ;-) > > > This is also required to make sure lazy callbacks in future patches > > > don't end up making rcu_barrier() wait for multiple seconds. > > > > But I do see that the wakeup is needed in the lazy case, and if I remember > > correctly, the ten-second rcu_barrier() delay really did happen. If I > > understand correctly, for this to happen, all of the callbacks must be > > in the bypass list, that is, ->cblist must be empty. > > > > So has the scenario steps 1-6 called out above actually happened in the > > absence of lazy callbacks? > > Nope, so I guess we can have the pending check around rcu_nocb_flush_bypass() > only... OK, sounds good. I have put this series on branch lazy.2022.10.14a and am testing it. Thanx, Paul
