Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state | Date | Fri, 27 May 2022 10:38:24 +0100 |
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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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