linux-kernel - [PATCH 1/3] rcu/nocb: Use switch/case on NOCB timer state machine

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Message-ID: <20241002145738.38226-2-frederic@kernel.org>
Date: Wed,  2 Oct 2024 16:57:36 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	rcu <rcu@...r.kernel.org>
Subject: [PATCH 1/3] rcu/nocb: Use switch/case on NOCB timer state machine

It's more convenient to benefit from the fallthrough feature of
switch / case to handle the timer state machine. Also a new state is
about to be added that will take advantage of it.

No intended functional change.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/rcu/tree_nocb.h | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 97b99cd06923..2fb803f863da 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -271,22 +271,35 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 
 	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
 
-	/*
-	 * Bypass wakeup overrides previous deferments. In case of
-	 * callback storms, no need to wake up too early.
-	 */
-	if (waketype == RCU_NOCB_WAKE_LAZY &&
-	    rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
-		mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
-		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
-	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
+	switch (waketype) {
+	case RCU_NOCB_WAKE_BYPASS:
+		/*
+		 * Bypass wakeup overrides previous deferments. In case of
+		 * callback storms, no need to wake up too early.
+		 */
 		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
 		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
-	} else {
+		break;
+	case RCU_NOCB_WAKE_LAZY:
+		if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
+			mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush());
+			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
+		}
+		/*
+		 * If the timer is already armed, a non-lazy enqueue may have happened
+		 * in-between. Don't delay it and fall-through.
+		 */
+		break;
+	case RCU_NOCB_WAKE:
+		fallthrough;
+	case RCU_NOCB_WAKE_FORCE:
 		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
 			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
 		if (rdp_gp->nocb_defer_wakeup < waketype)
 			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
+		break;
+	default:
+		WARN_ON_ONCE(1);
 	}
 
 	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
-- 
2.46.0