ftrace: Use synchronize_rcu_tasks_rude() instead of ftrace_sync()

This commit replaces the schedule_on_each_cpu(ftrace_sync) instances
with synchronize_rcu_tasks_rude().
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@redhat.com>
[ paulmck: Make Kconfig adjustments noted by kbuild test robot. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/trace/Kconfig

@@ -158,6 +158,7 @@ config FUNCTION_TRACER
 	select CONTEXT_SWITCH_TRACER
 	select GLOB
 	select TASKS_RCU if PREEMPTION
+	select TASKS_RUDE_RCU
 	help
 	  Enable the kernel to trace every kernel function. This is done
 	  by using a compiler feature to insert a small, 5-byte No-Operation

kernel/trace/ftrace.c

@@ -160,17 +160,6 @@ static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip,
 	op->saved_func(ip, parent_ip, op, regs);
 }
 
-static void ftrace_sync(struct work_struct *work)
-{
-	/*
-	 * This function is just a stub to implement a hard force
-	 * of synchronize_rcu(). This requires synchronizing
-	 * tasks even in userspace and idle.
-	 *
-	 * Yes, function tracing is rude.
-	 */
-}
-
 static void ftrace_sync_ipi(void *data)
 {
 	/* Probably not needed, but do it anyway */
@@ -256,7 +245,7 @@ static void update_ftrace_function(void)
 	 * Make sure all CPUs see this. Yes this is slow, but static
 	 * tracing is slow and nasty to have enabled.
 	 */
-	schedule_on_each_cpu(ftrace_sync);
+	synchronize_rcu_tasks_rude();
 	/* Now all cpus are using the list ops. */
 	function_trace_op = set_function_trace_op;
 	/* Make sure the function_trace_op is visible on all CPUs */
@@ -2932,7 +2921,7 @@ int ftrace_shutdown(struct ftrace_ops *ops, int command)
 		 * infrastructure to do the synchronization, thus we must do it
 		 * ourselves.
 		 */
-		schedule_on_each_cpu(ftrace_sync);
+		synchronize_rcu_tasks_rude();
 
 		/*
 		 * When the kernel is preeptive, tasks can be preempted
@@ -5887,7 +5876,7 @@ ftrace_graph_release(struct inode *inode, struct file *file)
 		 * infrastructure to do the synchronization, thus we must do it
 		 * ourselves.
 		 */
-		schedule_on_each_cpu(ftrace_sync);
+		synchronize_rcu_tasks_rude();
 
 		free_ftrace_hash(old_hash);
 	}