rcu/nocb: Only cancel nocb timer if not polling

This commit refrains deleting the ->nocb_timer if rcu_nocb is polling
because it should not ever have been queued in the polling case.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_plugin.h

@@ -2176,18 +2176,18 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
 	my_rdp->nocb_gp_gp = needwait_gp;
 	my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;
 	if (bypass) {
-		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
-		// Avoid race with first bypass CB.
-		if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
-			WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
-			del_timer(&my_rdp->nocb_timer);
-		}
 		if (!rcu_nocb_poll) {
+			raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
+			// Avoid race with first bypass CB.
+			if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
+				WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
+				del_timer(&my_rdp->nocb_timer);
+			}
 			// At least one child with non-empty ->nocb_bypass, so set
 			// timer in order to avoid stranding its callbacks.
 			mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
+			raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
 		}
-		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
 	}
 	if (rcu_nocb_poll) {
 		/* Polling, so trace if first poll in the series. */