On Fri, Oct 4, 2019 at 11:41 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote: > > On 10/04, Joel Fernandes (Google) wrote: > > > > But this is not always true if you consider the following events: > > I'm afraid I missed your point, but... > > > ----------------------> > > GP num 111111 22222222222222222222222222222222233333333 > > GP state i e p x r rx i > > CPU0 : rse rsx > > CPU1 : rse rsx > > CPU2 : rse rsx > > > > Here, we had 3 grace periods that elapsed, 1 for the rcu_sync_enter(), > > and 2 for the rcu_sync_exit(s). > > But this is fine? > > We only need to ensure that we have a full GP pass between the "last" > rcu_sync_exit() and GP_XXX -> GP_IDLE transition. > > > However, we had 3 rcu_sync_exit()s, not 2. In other words, the > > rcu_sync_exit() got batched. > > > > So my point here is, rcu_sync_exit() does not really always cause a new > > GP to happen > > See above, it should not. > > > Then what is the point of the GP_REPLAY state at all if it does not > > always wait for a new GP? > > Again, I don't understand... GP_REPLAY ensures that we will have a full GP > before rcu_sync_func() sets GP_IDLE, note that it does another "recursive" > call_rcu() if it sees GP_REPLAY. I finally got back to this (meanwhile life, job things happened). You are right, only the last one needs a full GP and it does get one here. Probably a comment in rcu_sync_exit() explaining this might help the future reader. Basically you are saying, if rcu_sync_exit() happens and GP_REPLAY is already set, we need not worry about starting a new GP because GP_REPLAY->GP_EXIT->GP_IDLE transition will involve a full GP anyway. And only if, GP_EXIT is already set, then we must set GP_REPLAY and wait for a full GP. This ensures the last rcu_sync_exit() gets a full GP. I think that was what I was missing. Some reason I thought that every rcu_sync_exit() needs to start a full GP. thanks! - Joel
