Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.37 as permitted sender) client-ip=23.128.96.37;
Message-ID: <tencent_D3E0324A65FA1569940651CEA0570820CB05@qq.com>
Date:   Wed, 25 Oct 2023 08:53:41 +0800
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH] stop_machine: pass curstate to ack_state()
Content-Language: en-US
From:   Rong Tao <rtoax@foxmail.com>
To:     open list <linux-kernel@vger.kernel.org>,
        Rong Tao <rongtao@cestc.cn>, rtoax@foxmail.com
References: <e2b302f9-5840-49e8-8176-c6c5ea725895@foxmail.com>
 <cdf3d6f1-5cfb-4bea-a19c-f523ec80eb68@foxmail.com>
In-Reply-To: <cdf3d6f1-5cfb-4bea-a19c-f523ec80eb68@foxmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


On 10/25/23 8:50 AM, Rong Tao wrote:
>
> 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()
OK, thanks a lot, i'll try that.
>
>>
>> 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