linux-kernel - [PATCH 06/16] rcu/nocb: Avoid confusing double write of rdp->nocb_cb

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Date:   Thu, 28 Jan 2021 18:12:12 +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>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Stable <stable@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 06/16] rcu/nocb: Avoid confusing double write of rdp->nocb_cb_sleep

rdp->nocb_cb_sleep is first set to true by default after processing
the callbacks then set back to false if we still find ready callbacks
to invoke.

This is confusing and even unsafe if it ever happens to be read
locklessly at some point. So make sure we write it only once per
nocb_cb_wait() loop.

Reported-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/rcu/tree_plugin.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 7781830a3cf1..53ff99a18ab1 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2241,6 +2241,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 	unsigned long flags;
 	bool needwake_state = false;
 	bool needwake_gp = false;
+	bool can_sleep = true;
 	struct rcu_node *rnp = rdp->mynode;
 
 	local_irq_save(flags);
@@ -2264,8 +2265,6 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
 	}
 
-	WRITE_ONCE(rdp->nocb_cb_sleep, true);
-
 	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
 		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)) {
 			rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_CB);
@@ -2273,7 +2272,7 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 				needwake_state = true;
 		}
 		if (rcu_segcblist_ready_cbs(cblist))
-			WRITE_ONCE(rdp->nocb_cb_sleep, false);
+			can_sleep = false;
 	} else {
 		/*
 		 * De-offloading. Clear our flag and notify the de-offload worker.
@@ -2286,6 +2285,8 @@ static void nocb_cb_wait(struct rcu_data *rdp)
 			needwake_state = true;
 	}
 
+	WRITE_ONCE(rdp->nocb_cb_sleep, can_sleep);
+
 	if (rdp->nocb_cb_sleep)
 		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
 
-- 
2.25.1