linux-kernel - Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target == state

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Date:   Fri, 27 May 2022 09:22:38 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/2] cpuhp: make target_store() a nop when target ==
 state

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