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.
Acked-by: Mark Rutland <[email protected]>
Signed-off-by: Rong Tao <[email protected]>
---
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
--
2.42.0
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 <[email protected]>
> Signed-off-by: Rong Tao <[email protected]>
> ---
> 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
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 <[email protected]>
>> Signed-off-by: Rong Tao <[email protected]>
>> ---
>> 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