On Fri, Jan 24, 2025 at 9:56 AM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote: > > Le Thu, Jan 23, 2025 at 08:49:47PM -0500, Joel Fernandes a écrit : > > On Thu, Dec 12, 2024 at 7:59 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > The get_state_synchronize_rcu_full() and poll_state_synchronize_rcu_full() > > > functions use the root rcu_node structure's ->gp_seq field to detect > > > the beginnings and ends of grace periods, respectively. This choice is > > > necessary for the poll_state_synchronize_rcu_full() function because > > > (give or take counter wrap), the following sequence is guaranteed not > > > to trigger: > > > > > > get_state_synchronize_rcu_full(&rgos); > > > synchronize_rcu(); > > > WARN_ON_ONCE(!poll_state_synchronize_rcu_full(&rgos)); > > > > > > The RCU callbacks that awaken synchronize_rcu() instances are > > > guaranteed not to be invoked before the root rcu_node structure's > > > ->gp_seq field is updated to indicate the end of the grace period. > > > However, these callbacks might start being invoked immediately > > > thereafter, in particular, before rcu_state.gp_seq has been updated. > > > Therefore, poll_state_synchronize_rcu_full() must refer to the > > > root rcu_node structure's ->gp_seq field. Because this field is > > > updated under this structure's ->lock, any code following a call to > > > poll_state_synchronize_rcu_full() will be fully ordered after the > > > full grace-period computation, as is required by RCU's memory-ordering > > > semantics. > > > > > > By symmetry, the get_state_synchronize_rcu_full() function should also > > > use this same root rcu_node structure's ->gp_seq field. But it turns out > > > that symmetry is profoundly (though extremely infrequently) destructive > > > in this case. To see this, consider the following sequence of events: > > > > > > 1. CPU 0 starts a new grace period, and updates rcu_state.gp_seq > > > accordingly. > > I don't think so because idle CPUs are waited upon to report a QS, unlike > offline CPUs that don't appear in ->qsmaskinitnext. > > If the CPU 1 is idle while the grace period kthread scans its > ct_rcu_watching_cpu(), then the QS is reported on its behalf and when CPU 1 > goes out of idle it is guaranteed to see the new started GP on the root node. > > If the CPU 1 is not idle while the grace period kthread scans its > ct_rcu_watching_cpu(), then CPU 1 must report a QS and that cancels the race. Sorry, you are right. Idle CPUs are required to have QS reports done for them or on their behalf. My bad. thanks, - Joel
