rcu: Comment rcu_gp_init() code waiting for CPU-hotplug operations

Near the beginning of rcu_gp_init() is a per-rcu_node loop that waits
for CPU-hotplug operations that might have started before the new
grace period did.  This commit adds a comment explaining that this
wait does not exclude CPU-hotplug operations.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0bfebec..e6e1b92 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1775,6 +1775,8 @@ static noinline_for_stack bool rcu_gp_init(void)
         */
        WRITE_ONCE(rcu_state.gp_state, RCU_GP_ONOFF);
        rcu_for_each_leaf_node(rnp) {
+               // Wait for CPU-hotplug operations that might have
+               // started before this grace period did.
                smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.
                firstseq = READ_ONCE(rnp->ofl_seq);
                if (firstseq & 0x1)