[PATCH RFC ftrace] Chose RCU Tasks based on TASKS_RCU rather than PREEMPTION

[Paul E. McKenney]

The advent of CONFIG_PREEMPT_AUTO, AKA lazy preemption, will mean that
even kernels built with CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY
might see the occasional preemption, and that this preemption just might
happen within a trampoline.

Therefore, update ftrace_shutdown() to invoke synchronize_rcu_tasks()
based on CONFIG_TASKS_RCU instead of CONFIG_PREEMPTION.

Only build tested.

Signed-off-by: Paul E. McKenney <paul...@kernel.org>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Masami Hiramatsu <mhira...@kernel.org>
Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Cc: Ankur Arora <ankur.a.ar...@oracle.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: <linux-trace-ker...@vger.kernel.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 2da4eaa2777d6..c9e6c69cf3446 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3156,7 +3156,7 @@ int ftrace_shutdown(struct ftrace_ops *ops, int command)
                 * synchronize_rcu_tasks() will wait for those tasks to
                 * execute and either schedule voluntarily or enter user space.
                 */
-               if (IS_ENABLED(CONFIG_PREEMPTION))
+               if (IS_ENABLED(CONFIG_TASKS_RCU))
                        synchronize_rcu_tasks();
 
                ftrace_trampoline_free(ops);