[RFC PATCH 08/12] rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY

[Frederic Weisbecker]

Make sure to handle the pending bypass queue before we switch to the
final de-offload state. We'll have to be careful and later set
SEGCBLIST_SOFTIRQ_ONLY before re-enabling again IRQs, or new bypass
callbacks could be queued in the meantine.

Inspired-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3361bd351f3b..cee15841c178 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2302,11 +2302,17 @@ static void __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 			      !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB |
 							SEGCBLIST_KTHREAD_GP));
 
+	rcu_nocb_lock_irqsave(rdp, flags);
 	/* Make sure nocb timer won't stay around */
-	rcu_nocb_lock_irqsave(rdp, flags);
 	WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_OFF);
-	rcu_nocb_unlock_irqrestore(rdp, flags);
 	del_timer_sync(&rdp->nocb_timer);
+	/*
+	 * Flush bypass. While IRQs are disabled and once we set
+	 * SEGCBLIST_SOFTIRQ_ONLY, no callback is supposed to be
+	 * enqueued on bypass.
+	 */
+	rcu_nocb_flush_bypass(rdp, NULL, jiffies);
+	rcu_nocb_unlock_irqrestore(rdp, flags);
 }
 
 static long rcu_nocb_rdp_deoffload(void *arg)
-- 
2.28.0