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Date:   Wed, 25 Oct 2023 08:50:16 +0800
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Subject: Re: [PATCH] stop_machine: pass curstate to ack_state()
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To:     open list <linux-kernel@vger.kernel.org>,
        Rong Tao <rongtao@cestc.cn>
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From:   Rong Tao <rtoax@foxmail.com>
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On 10/25/23 8:43 AM, Rong Tao wrote:
> The multi_cpu_stop() state machine uses multi_stop_data::state to hold
> the current state, and this is read and written atomically except in
> ack_state(), which performs a non-atomic read.
>
> As ack_state() only performs this non-atomic read when there is a single
> writer, this is benign, but it makes reasoning about the state machine a
> little harder.
>
> Remove the non-atomic read and pass the (atomically read) curstate in
> instead. This makes it clear that we do not expect any racy writes, and
> avoids a redundant load.

I'd prefer if we make this comment:

stop_machine: pass curstate to ack_state()

>
> Acked-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
>  kernel/stop_machine.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index cedb17ba158a..268c2e581698 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -188,10 +188,11 @@ static void set_state(struct multi_stop_data 
> *msdata,
>  }
>
>  /* Last one to ack a state moves to the next state. */
> -static void ack_state(struct multi_stop_data *msdata)
> +static void ack_state(struct multi_stop_data *msdata,
> +              enum multi_stop_state curstate)
>  {
>      if (atomic_dec_and_test(&msdata->thread_ack))
> -        set_state(msdata, msdata->state + 1);
> +        set_state(msdata, curstate + 1);
>  }
>
>  notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
> @@ -242,7 +243,7 @@ static int multi_cpu_stop(void *data)
>              default:
>                  break;
>              }
> -            ack_state(msdata);
> +            ack_state(msdata, curstate);
>          } else if (curstate > MULTI_STOP_PREPARE) {
>              /*
>               * At this stage all other CPUs we depend on must spin