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

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Date:   Wed, 25 May 2022 10:48:55 +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] cpuhp: make target_store() a nop when target == state

On 24/05/22 15:37, Phil Auld wrote:
> Hi Valentin,
>
> I did it like this (shown below) and from my test it also works for
> this case.
>
> I could move it below the lock and goto out;  instead if you think
> that is better.

I *think* the cpu_add_remove_lock mutex should be sufficient here.

> It still seems better to me to stop this higher up
> because there's work being done in the out path too.  We're not
> actually doing any hot(un)plug so doing post unplug cleanup seems
> iffy.
>

I think so too; I now realize _cpu_up() and _cpu_down() have slightly
different prologues: _cpu_up() does its hotplug states / cpu_present_mask
checks *after* grabbing the cpu_hotplug_lock, _cpu_down() does that *before*...

So I believe what you have below is fine, modulo whether we want to align
the prologue of these two functions or not :-)

> _cpu_down()
> ...
> out:
>         cpus_write_unlock();
>         /*
>          * Do post unplug cleanup. This is still protected against
>          * concurrent CPU hotplug via cpu_add_remove_lock.
>          */
>         lockup_detector_cleanup();
>         arch_smt_update();
>         cpu_up_down_serialize_trainwrecks(tasks_frozen);
>       return ret;
> }
>
> ----------
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 8a71b1149c60..e36788742d18 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1130,6 +1130,13 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
>       if (!cpu_present(cpu))
>               return -EINVAL;
>
> +	/*
> +	 * The caller of cpu_down() might have raced with another
> +	 * caller. Nothing to do.
> +	 */
> +	if (st->state <= target)
> +		return 0;
> +
>       cpus_write_lock();
>
>       cpuhp_tasks_frozen = tasks_frozen;
>
>
>
>
> Cheers,
> Phil
>
> --