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From:   "Joel Fernandes (Google)" <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        Steven Rostedt <rostedt@goodmis.org>
Subject: [PATCH] rcu/segcblist: Add debug check for whether seglen is 0 for empty list
Date:   Wed, 18 Nov 2020 11:10:43 -0500
Message-Id: <20201118161043.3823994-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

After rcu_do_batch(), add a check for whether the seglen counts went to
zero if the list was indeed empty.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
 kernel/rcu/rcu_segcblist.c | 12 ++++++++++++
 kernel/rcu/rcu_segcblist.h |  3 +++
 kernel/rcu/tree.c          |  1 +
 3 files changed, 16 insertions(+)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 5059b6102afe..6e98bb3804f0 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -94,6 +94,18 @@ static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
 	return READ_ONCE(rsclp->seglen[seg]);
 }
 
+/* Return number of callbacks in segmented callback list by totalling seglen. */
+long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp)
+{
+	long len = 0;
+	int i;
+
+	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
+		len += rcu_segcblist_get_seglen(rsclp, i);
+
+	return len;
+}
+
 /* Set the length of a segment of the rcu_segcblist structure. */
 static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
 {
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index cd35c9faaf51..46a42d77f7e1 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -15,6 +15,9 @@ static inline long rcu_cblist_n_cbs(struct rcu_cblist *rclp)
 	return READ_ONCE(rclp->len);
 }
 
+/* Return number of callbacks in segmented callback list by totalling seglen. */
+long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp);
+
 void rcu_cblist_init(struct rcu_cblist *rclp);
 void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp);
 void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f5b61e10f1de..928bd10c9c3b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2553,6 +2553,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	WARN_ON_ONCE(count == 0 && !rcu_segcblist_empty(&rdp->cblist));
 	WARN_ON_ONCE(!IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
 		     count != 0 && rcu_segcblist_empty(&rdp->cblist));
+	WARN_ON_ONCE(count == 0 && !rcu_segcblist_n_segment_cbs(&rdp->cblist));
 
 	rcu_nocb_unlock_irqrestore(rdp, flags);
 
-- 
2.29.2.299.gdc1121823c-goog