On Thu, Jun 27, 2024 at 01:36:58PM +0200, Frederic Weisbecker wrote:
> When the grace period kthread checks the extended quiescent state
> counter of a CPU, full ordering is necessary to ensure that either:
Just to make sure that I understand...
I need to replace these commits in -rcu:
da979d0162fc6 rcu: Remove full ordering on second EQS snapshot
6411f4185f657 rcu: Remove superfluous full memory barrier upon first EQS snapshot
dec56ca5f1c34 rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot
With these three patches, and keep these three commits as they are?
d43a302fc08a5 rcu: Remove full memory barrier on boot time eqs sanity check
b1c36aa90cbf1 rcu: Remove full memory barrier on RCU stall printout
64d68f1d53f77 rcu/exp: Remove redundant full memory barrier at the end of GP
Thanx, Paul
> * If the GP kthread observes the remote target in an extended quiescent
> state, then that target must observe all accesses prior to the current
> grace period, including the current grace period sequence number, once
> it exits that extended quiescent state.
>
> or:
>
> * If the GP kthread observes the remote target NOT in an extended
> quiescent state, then the target further entering in an extended
> quiescent state must observe all accesses prior to the current
> grace period, including the current grace period sequence number, once
> it enters that extended quiescent state.
>
> This ordering is enforced through a full memory barrier placed right
> before taking the first EQS snapshot. However this is superfluous
> because the snapshot is taken while holding the target's rnp lock which
> provides the necessary ordering through its chain of
> smp_mb__after_unlock_lock().
>
> Remove the needless explicit barrier before the snapshot and put a
> comment about the implicit barrier newly relied upon here.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> Reviewed-by: Neeraj Upadhyay <neeraj.upadhyay@xxxxxxxxxx>
> ---
> kernel/rcu/tree_exp.h | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8a1d9c8bd9f7..1dbad2442e8d 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -357,7 +357,21 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
> !(rnp->qsmaskinitnext & mask)) {
> mask_ofl_test |= mask;
> } else {
> - snap = rcu_dynticks_snap(cpu);
> + /*
> + * Full ordering between remote CPU's post idle accesses
> + * and updater's accesses prior to current GP (and also
> + * the started GP sequence number) is enforced by
> + * rcu_seq_start() implicit barrier, relayed by kworkers
> + * locking and even further by smp_mb__after_unlock_lock()
> + * barriers chained all the way throughout the rnp locking
> + * tree since sync_exp_reset_tree() and up to the current
> + * leaf rnp locking.
> + *
> + * Ordering between remote CPU's pre idle accesses and
> + * post grace period updater's accesses is enforced by the
> + * below acquire semantic.
> + */
> + snap = ct_dynticks_cpu_acquire(cpu);
> if (rcu_dynticks_in_eqs(snap))
> mask_ofl_test |= mask;
> else
> --
> 2.45.2
>
