[PATCH RFC] stop_machine: make stop_machine safe and efficient to call early

[Jeremy Fitzhardinge <jeremy@goop.org>]

Make stop_machine() safe to call early in boot, before SMP has been
set up, by simply calling the callback function directly if there's
only one CPU online.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index ba5070c..2df15ca 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -485,6 +485,9 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 					    .num_threads = num_online_cpus(),
 					    .active_cpus = cpus };
 
+	if (smdata.num_threads == 1)
+		return (*fn)(data);
+
 	/* Set the initial state and stop all online cpus. */
 	set_state(&smdata, STOPMACHINE_PREPARE);
 	return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);