Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] cpuhp: make target_store() a nop when target == state | Date | Tue, 24 May 2022 16:11:51 +0100 |
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On 23/05/22 10:47, Phil Auld wrote: > writing the current state back into 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); > > Make this safe by catching the case where target == state > and bailing early. > > Signed-off-by: Phil Auld <pauld@redhat.com> > --- > > Yeah, I know... don't do that. But it's still messy. > > !< != > > > kernel/cpu.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index d0a9aa0b42e8..8a71b1149c60 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -2302,6 +2302,9 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr, > return -EINVAL; > #endif > > + if (target == st->state) > + return count; > +
The current checks are against static boundaries, this has to compare against st->state - AFAICT this could race with another hotplug operation to the same CPU, e.g.
CPU42.cpuhp_state ->state == CPUHP_AP_SCHED_STARTING ->target == CPUHP_ONLINE
<write CPUHP_ONLINE via sysfs, OK because current state != CPUHP_ONLINE>
CPU42.cpuhp_state == CPUHP_ONLINE
<issues ensue>
_cpu_up() has:
/* * The caller of cpu_up() might have raced with another * caller. Nothing to do. */ if (st->state >= target) goto out;
Looks like we want an equivalent in _cpu_down(), what do you think?
> ret = lock_device_hotplug_sysfs(); > if (ret) > return ret; > -- > 2.18.0
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