[PATCH 6.1 0164/1146] cpu/hotplug: Make target_store() a nop when target == state

Greg Kroah-Hartman
2022-12-28T15:19:48Z

From: Phil Auld <pauld@redhat.com>

[ Upstream commit 64ea6e44f85b9b75925ebe1ba0e6e8430cc4e06f ]

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@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20221117162329.3164999-2-pauld@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/cpu.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bbad5e375d3b..979de993f853 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;
--
2.35.1