[tip: smp/core] cpuhp: Make target_store() a nop when target == state

[tip-bot2 for Phil Auld]

The following commit has been merged into the smp/core branch of tip:

Commit-ID:     0fa5abb6b7d85bf5688b2e11113f50317fb0121c
Gitweb:        https://git.kernel.org/tip/0fa5abb6b7d85bf5688b2e11113f50317fb0121c
Author:        Phil Auld <pauld@xxxxxxxxxx>
AuthorDate:    Thu, 17 Nov 2022 11:23:28 -05:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 01 Dec 2022 12:35:08 +01:00

cpuhp: Make target_store() a nop when target == state

Writing the current state back in 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);

Fix this by letting state==target fall through in the target_store()
conditional. Also make sure st->target == target in that case.

Fixes: 757c989b9994 ("cpu/hotplug: Make target state writeable")
Signed-off-by: Phil Auld <pauld@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20221117162329.3164999-2-pauld@xxxxxxxxxx

---
 kernel/cpu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bbad5e3..979de99 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
 
 	if (st->state < target)
 		ret = cpu_up(dev->id, target);
-	else
+	else if (st->state > target)
 		ret = cpu_down(dev->id, target);
+	else if (WARN_ON(st->target != target))
+		st->target = target;
 out:
 	unlock_device_hotplug();
 	return ret ? ret : count;