rcu-tasks: Handle sparse cpu_possible_mask in rcu_tasks_invoke_cbs()

If the cpu_possible_mask is sparse (for example, if bits are set only for
CPUs 0, 4, 8, ...), then rcu_tasks_invoke_cbs() will access per-CPU data
for a CPU not in cpu_possible_mask.  It makes these accesses while doing
a workqueue-based binary search for non-empty callback lists.  Although
this search must pass through CPUs not represented in cpu_possible_mask,
it has no need to check the callback list for such CPUs.

This commit therefore changes the rcu_tasks_invoke_cbs() function's
binary search so as to only check callback lists for CPUs present in
cpu_possible_mask.

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -468,7 +468,7 @@ static void rcu_tasks_invoke_cbs(struct rcu_tasks *rtp, struct rcu_tasks_percpu
 		}
 	}
 
-	if (rcu_segcblist_empty(&rtpcp->cblist))
+	if (rcu_segcblist_empty(&rtpcp->cblist) || !cpu_possible(cpu))
 		return;
 	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
 	rcu_segcblist_advance(&rtpcp->cblist, rcu_seq_current(&rtp->tasks_gp_seq));