[PATCH] cpuhp: make target_store() a nop when target == state

[Phil Auld <pauld@redhat.com>]

writing the current state back into hotplug/target calls cpu_down()
which will set cpu dying even when it isn't and then nothing will
ever clear it. A stress test that reads values and writes them back
for all cpu device files in sysfs will trigger the BUG() in
select_fallback_rq once all cpus are marked as dying.

kernel/cpu.c::target_store()
	...
        if (st->state < target)
                ret = cpu_up(dev->id, target);
        else
                ret = cpu_down(dev->id, target);

cpu_down() -> cpu_set_state()
	 bool bringup = st->state < target;
	 ...
	 if (cpu_dying(cpu) != !bringup)
		set_cpu_dying(cpu, !bringup);

Make this safe by catching the case where target == state
and bailing early.

Signed-off-by: Phil Auld <pauld@redhat.com>
---

Yeah, I know... don't do that. But it's still messy.

!< != > 

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

diff --git a/kernel/cpu.c b/kernel/cpu.c
index d0a9aa0b42e8..8a71b1149c60 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2302,6 +2302,9 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
 		return -EINVAL;
 #endif
 
+	if (target == st->state)
+		return count;
+
 	ret = lock_device_hotplug_sysfs();
 	if (ret)
 		return ret;
-- 
2.18.0