[RFC PATCH 07/12] rcu: Shutdown nocb timer on de-offloading

[Frederic Weisbecker <frederic@kernel.org>]

Make sure the nocb timer can't fire anymore before we reach the final
de-offload state. Spuriously waking up the GP kthread is no big deal but
we must prevent from executing the timer callback without nocb locking.

Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
 kernel/rcu/tree.h        |  1 +
 kernel/rcu/tree_plugin.h | 12 +++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 5b37f7103b0d..ca69238e2227 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -254,6 +254,7 @@ struct rcu_data {
 };
 
 /* Values for nocb_defer_wakeup field in struct rcu_data. */
+#define RCU_NOCB_WAKE_OFF	-1
 #define RCU_NOCB_WAKE_NOT	0
 #define RCU_NOCB_WAKE		1
 #define RCU_NOCB_WAKE_FORCE	2
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index eceef6dade9a..3361bd351f3b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1637,6 +1637,8 @@ static void wake_nocb_gp(struct rcu_data *rdp, bool force,
 static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
 			       const char *reason)
 {
+	if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_OFF)
+		return;
 	if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
 		mod_timer(&rdp->nocb_timer, jiffies + 1);
 	if (rdp->nocb_defer_wakeup < waketype)
@@ -2214,7 +2216,7 @@ static int rcu_nocb_cb_kthread(void *arg)
 /* Is a deferred wakeup of rcu_nocb_kthread() required? */
 static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp)
 {
-	return READ_ONCE(rdp->nocb_defer_wakeup);
+	return READ_ONCE(rdp->nocb_defer_wakeup) > RCU_NOCB_WAKE_NOT;
 }
 
 /* Do a deferred wakeup of rcu_nocb_kthread(). */
@@ -2299,6 +2301,12 @@ static void __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));
+
+	/* 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);
 }
 
 static long rcu_nocb_rdp_deoffload(void *arg)
@@ -2344,6 +2352,8 @@ static void __rcu_nocb_rdp_offload(struct rcu_data *rdp)
 	 * SEGCBLIST_SOFTIRQ_ONLY mode.
 	 */
 	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+	/* Re-enable nocb timer */
+	WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
 	/*
 	 * We didn't take the nocb lock while working on the
 	 * rdp->cblist in SEGCBLIST_SOFTIRQ_ONLY mode.
-- 
2.28.0