bpf: Skip rcu_barrier() if rcu_trace_implies_rcu_gp() is true

If there are pending rcu callback, free_mem_alloc() will use
rcu_barrier_tasks_trace() and rcu_barrier() to wait for the pending
__free_rcu_tasks_trace() and __free_rcu() callback.

If rcu_trace_implies_rcu_gp() is true, there will be no pending
__free_rcu(), so it will be OK to skip rcu_barrier() as well.
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20221209010947.3130477-3-houtao@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/memalloc.c

@@ -464,9 +464,17 @@ static void free_mem_alloc(struct bpf_mem_alloc *ma)
 {
 	/* waiting_for_gp lists was drained, but __free_rcu might
 	 * still execute. Wait for it now before we freeing percpu caches.
+	 *
+	 * rcu_barrier_tasks_trace() doesn't imply synchronize_rcu_tasks_trace(),
+	 * but rcu_barrier_tasks_trace() and rcu_barrier() below are only used
+	 * to wait for the pending __free_rcu_tasks_trace() and __free_rcu(),
+	 * so if call_rcu(head, __free_rcu) is skipped due to
+	 * rcu_trace_implies_rcu_gp(), it will be OK to skip rcu_barrier() by
+	 * using rcu_trace_implies_rcu_gp() as well.
 	 */
 	rcu_barrier_tasks_trace();
-	rcu_barrier();
+	if (!rcu_trace_implies_rcu_gp())
+		rcu_barrier();
 	free_mem_alloc_no_barrier(ma);
 }