[PATCH 02/16] rcu/nocb: Comment the reason behind BH disablement on batch processing

[Frederic Weisbecker]

Explain why we need to disable softirqs while processing callbacks in
an offline fashion. The subtle reason doesn't want to be forgotten.

Reported-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index a44f80d7661b..dcfae03eb9e9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2235,6 +2235,12 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	local_irq_save(flags);
 	rcu_momentary_dyntick_idle();
 	local_irq_restore(flags);
+	/*
+	 * While transitioning to/from NOCB mode, a CPU might execute the same
+	 * callback concurrently if it requeues itself and the softirq interrupts
+	 * the offloaded callback processing. Make sure we disable BH to prevent
+	 * from that.
+	 */
 	local_bh_disable();
 	rcu_do_batch(rdp);
 	local_bh_enable();
-- 
2.25.1