linux-kernel - [PATCH 09/19] rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ

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Date:   Fri, 13 Nov 2020 13:13:24 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>, rcu@...r.kernel.org
Subject: [PATCH 09/19] rcu: Flush bypass before setting SEGCBLIST_SOFTIRQ_ONLY

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@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree_plugin.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 44b4ab9b3953..dfb4b62c6b88 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2339,12 +2339,21 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	swait_event_exclusive(rdp->nocb_state_wq,
 			      !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_lock_irqsave(rdp, flags);
+	rcu_nocb_flush_bypass(rdp, NULL, jiffies);
+	rcu_nocb_unlock_irqrestore(rdp, flags);
+
 	return ret;
 }
 
-- 
2.25.1