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From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Zqiang <qiang1.zhang@intel.com>
Cc:     rcu@vger.kernel.org
Subject: [PATCH v2 09/13] rcu: Remove never-set needwake assignment from rcu_report_qs_rdp()
Date:   Sat, 25 Mar 2023 17:33:12 +0000
Message-Id: <20230325173316.3118674-10-joel@joelfernandes.org>
In-Reply-To: <20230325173316.3118674-1-joel@joelfernandes.org>
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From: Zqiang <qiang1.zhang@intel.com>

The rcu_accelerate_cbs() function is invoked by rcu_report_qs_rdp()
only if there is a grace period in progress that is still blocked
by at least one CPU on this rcu_node structure.  This means that
rcu_accelerate_cbs() should never return the value true, and thus that
this function should never set the needwake variable and in turn never
invoke rcu_gp_kthread_wake().

This commit therefore removes the needwake variable and the invocation
of rcu_gp_kthread_wake() in favor of a WARN_ON_ONCE() on the call to
rcu_accelerate_cbs().  The purpose of this new WARN_ON_ONCE() is to
detect situations where the system's opinion differs from ours.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8e880c09ab59..e80e8f128c57 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1955,7 +1955,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
 {
 	unsigned long flags;
 	unsigned long mask;
-	bool needwake = false;
 	bool needacc = false;
 	struct rcu_node *rnp;
 
@@ -1987,7 +1986,12 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
 		 * NOCB kthreads have their own way to deal with that...
 		 */
 		if (!rcu_rdp_is_offloaded(rdp)) {
-			needwake = rcu_accelerate_cbs(rnp, rdp);
+			/*
+			 * The current GP has not yet ended, so it
+			 * should not be possible for rcu_accelerate_cbs()
+			 * to return true.  So complain, but don't awaken.
+			 */
+			WARN_ON_ONCE(rcu_accelerate_cbs(rnp, rdp));
 		} else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
 			/*
 			 * ...but NOCB kthreads may miss or delay callbacks acceleration
@@ -1999,8 +2003,6 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
 		rcu_disable_urgency_upon_qs(rdp);
 		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
 		/* ^^^ Released rnp->lock */
-		if (needwake)
-			rcu_gp_kthread_wake();
 
 		if (needacc) {
 			rcu_nocb_lock_irqsave(rdp, flags);
-- 
2.40.0.348.gf938b09366-goog