[PATCH 4.12 196/196] smp/hotplug: Replace BUG_ON and react useful

[Greg Kroah-Hartman]

4.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit dea1d0f5f1284e3defee4b8484d9fc230686cd42 upstream.

The move of the unpark functions to the control thread moved the BUG_ON()
there as well. While it made some sense in the idle thread of the upcoming
CPU, it's bogus to crash the control thread on the already online CPU,
especially as the function has a return value and the callsite is prepared
to handle an error return.

Replace it with a WARN_ON_ONCE() and return a proper error code.

Fixes: 9cd4f1a4e7a8 ("smp/hotplug: Move unparking of percpu threads to the control CPU")
Rightfully-ranted-at-by: Linux Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/cpu.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -354,7 +354,8 @@ static int bringup_wait_for_ap(unsigned
 
 	/* Wait for the CPU to reach CPUHP_AP_ONLINE_IDLE */
 	wait_for_completion(&st->done);
-	BUG_ON(!cpu_online(cpu));
+	if (WARN_ON_ONCE((!cpu_online(cpu))))
+		return -ECANCELED;
 
 	/* Unpark the stopper thread and the hotplug thread of the target cpu */
 	stop_machine_unpark(cpu);