rcu: Remove _rcu_barrier() dependency on __stop_machine()

Currently, _rcu_barrier() relies on preempt_disable() to prevent
any CPU from going offline, which in turn depends on CPU hotplug's
use of __stop_machine().

This patch therefore makes _rcu_barrier() use get_online_cpus() to
block CPU-hotplug operations.  This has the added benefit of removing
the need for _rcu_barrier() to adopt callbacks:  Because CPU-hotplug
operations are excluded, there can be no callbacks to adopt.  This
commit simplifies the code accordingly.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index abffb486e94e..6a2e52a85d77 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -51,8 +51,8 @@ static int show_rcubarrier(struct seq_file *m, void *unused)
         struct rcu_state *rsp;
 
         for_each_rcu_flavor(rsp)
-                seq_printf(m, "%s: %c bcc: %d nbd: %lu\n",
-                           rsp->name, rsp->rcu_barrier_in_progress ? 'B' : '.',
+                seq_printf(m, "%s: bcc: %d nbd: %lu\n",
+                           rsp->name,
                            atomic_read(&rsp->barrier_cpu_count),
                            rsp->n_barrier_done);
         return 0;