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Date:   Tue, 19 Jul 2022 11:03:10 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Valentin Schneider <vschneid@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 1/2] cpuhp: make target_store() a nop when target ==
 state
Message-ID: <YtbHrmDem1v++dBL@lorien.usersys.redhat.com>
References: <20220711211619.112854-1-pauld@redhat.com>
 <20220711211619.112854-2-pauld@redhat.com>
 <xhsmhk0896ujo.mognet@vschneid.remote.csb>
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On Tue, Jul 19, 2022 at 03:34:03PM +0100 Valentin Schneider wrote:
> On 11/07/22 17:16, 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. Also make sure st->target == target in that case.
> >
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> 
> One nit below, otherwise:
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
>

Thanks!

> > ---
> >  kernel/cpu.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index bbad5e375d3b..305694a2ca26 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 (st->target != target)
> 
> Should we make this:
> 
>         else if (WARN(st->target != target))
>

If you think that's important I can make it a WARN, sure.

I'll try to remember to keep your Reviewed-bys this time if that's okay.


Cheers,
Phil

> > +		st->target = target;
> >  out:
> >       unlock_device_hotplug();
> >       return ret ? ret : count;
> > --
> > 2.31.1
> 

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