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

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Message-ID: <xhsmho7zf8auj.mognet@vschneid.remote.csb>
Date:   Mon, 30 May 2022 13:27:00 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Phil Auld <pauld@...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 27/05/22 09:22, Phil Auld wrote:
> 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?
>

Yeah, you could append a simple:

        else
                WARN_ON(st->state != target);


> Maybe I'll include that if/when I have code to keep cpux/online in sync
> with st->state and cpu_online_mask.