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SubjectRe: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state
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On 26/05/22 12:06, Phil Auld wrote:
> 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.
>

To go back on my data race paranoia: writes to both cpu$x/online and
cpu$x/hotplug/target are serialized by device_hotplug_lock, and so are the
exported kernel hotplug functions ({add, remove}_cpu()).

That's not cpu_add_remove_lock as I was looking for, but that's still all
under one lock, so I think we're good. Sorry for that!

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

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

> ---
> kernel/cpu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index d0a9aa0b42e8..cdb6ac10ad94 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2315,7 +2315,7 @@ 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);
> out:
> unlock_device_hotplug();
> --
> 2.18.0

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