Hi Oleg, On Fri, Oct 04, 2019 at 05:41:03PM +0200, Oleg Nesterov 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. Ok, I understand now. The point is to wait for a full GP, not necessarily start a new one on each exit. > > 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. Ok, got it. > > Taking a step back, why did we intend to have > > to wait for a new GP if another rcu_sync_exit() comes while one is still > > in progress? > > To ensure that if another CPU sees rcu_sync_is_idle() (GP_IDLE) after you > do rcu_sync_exit(), then it must also see all memory changes you did before > rcu_sync_exit(). Would this not be better implemented using memory barriers, than starting new grace periods just for memory ordering? A memory barrier is lighter than having to go through a grace period. So something like: if the state is already GP_EXIT, then rcu_sync_exit() issues a memory barrier instead of replaying. But if state is GP_PASSED, then wait for a grace period. Or, do you see a situation where this will not work? thanks, - Joel
