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

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Date:   Tue, 24 May 2022 12:39:33 -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] cpuhp: make target_store() a nop when target == state

On Tue, May 24, 2022 at 04:11:51PM +0100 Valentin Schneider wrote:
> 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@...hat.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>
>

What I'm trying to fix is not a race.  It's just bogus logic. 
There is an assumption here that !< means > which is just not
true. 

This potential race seems orthogonal and not even effected
one way or the other by this code change, right?

I could not convince myself that the check I added needed to
be under the locks because returning success when the state
is already reporting what you asked for seems harmless.


> 
> _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?

Maybe. I still think that

> >         if (st->state < target)
> >                 ret = cpu_up(dev->id, target);
> >         else
> >                 ret = cpu_down(dev->id, target);

is not correct. If we catch the == case earlier then this makes
sense as is.

I suppose "if (st->state <= target)" would work too since __cpu_up()
already checks. Catching this sooner seems better to me though.

> 
> >  	ret = lock_device_hotplug_sysfs();
> >  	if (ret)
> >  		return ret;
> > -- 
> > 2.18.0
> 


Cheers,
Phil

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