rcu: update documentation/comments for Lai's adoption patch

Lai's RCU-callback immediate-adoption patch changes the RCU tracing
output, so update tracing.txt.  Also update a few comments to clarify
the synchronization design.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 669d7fe049d1..120820ffc657 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1668,7 +1668,9 @@ static void _rcu_barrier(struct rcu_state *rsp,
          * decrement rcu_barrier_cpu_count -- otherwise the first CPU
          * might complete its grace period before all of the other CPUs
          * did their increment, causing this function to return too
-         * early.
+         * early.  Note that on_each_cpu() disables irqs, which prevents
+         * any CPUs from coming online or going offline until each online
+         * CPU has queued its RCU-barrier callback.
          */
         atomic_set(&rcu_barrier_cpu_count, 1);
         on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
@@ -1797,9 +1799,9 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
         case CPU_DYING:
         case CPU_DYING_FROZEN:
                 /*
-                 * The whole machine is "stopped" except this cpu, so we can
-                 * touch any data without introducing corruption. And we send
-                 * the callbacks to an attribute chosen online cpu.
+                 * The whole machine is "stopped" except this CPU, so we can
+                 * touch any data without introducing corruption. We send the
+                 * dying CPU's callbacks to an arbitrarily chosen online CPU.
                  */
                 rcu_send_cbs_to_online(&rcu_bh_state);
                 rcu_send_cbs_to_online(&rcu_sched_state);