linux-kernel - [PATCH RFC ftrace] Chose RCU Tasks based on TASKS

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Date: Wed, 28 Feb 2024 11:38:29 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ankur Arora <ankur.a.arora@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>, kernel-team@...a.com
Subject: [PATCH RFC ftrace] Chose RCU Tasks based on TASKS_RCU rather than
 PREEMPTION

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 <paulmck@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: <linux-trace-kernel@...r.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);