Messages in this thread | | | Date | Fri, 27 May 2022 09:22:38 -0400 | From | Phil Auld <> | Subject | Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state |
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On Fri, May 27, 2022 at 10:38:24AM +0100 Valentin Schneider wrote: > 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! >
Right. This catches it up higher so that we don't get into the code that starts actually changing things. I wonder now in the state == target case if we should make sure st->target == target. With the second patch it's less likely to be needed. Thoughts?
Maybe I'll include that if/when I have code to keep cpux/online in sync with st->state and cpu_online_mask.
> > Signed-off-by: Phil Auld <pauld@redhat.com> > > Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Thanks!
Cheers, Phil
> > > --- > > 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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