stop_machine, rcu: Mark functions as notrace

Some architectures assume that the stopped CPUs don't make function calls
to traceable functions when they are in the stopped state. See also commit
cb9d7fd51d9f ("watchdog: Mark watchdog touch functions as notrace").

Violating this assumption causes kernel crashes when switching tracer on
RISC-V.

Mark rcu_momentary_dyntick_idle() and stop_machine_yield() notrace to
prevent this.

Fixes: 4ecf0a43e729 ("processor: get rid of cpu_relax_yield")
Fixes: 366237e7b083 ("stop_machine: Provide RCU quiescent state in multi_cpu_stop()")
Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201021073839.43935-1-zong.li@sifive.com
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 06895ef..2a52f42 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -409,7 +409,7 @@
  *
  * The caller must have disabled interrupts and must not be idle.
  */
-void rcu_momentary_dyntick_idle(void)
+notrace void rcu_momentary_dyntick_idle(void)
 {
 	int special;
 
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 865bb02..890b79cf 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -178,7 +178,7 @@
 		set_state(msdata, msdata->state + 1);
 }
 
-void __weak stop_machine_yield(const struct cpumask *cpumask)
+notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
 {
 	cpu_relax();
 }