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. Thanks.
