From: Rong Tao <[email protected]>
commit b1fc58333575 ("stop_machine: Avoid potential race behaviour")
solved the race behaviour problem, to better show that race behaviour
does not exist, pass the 'curstate' directly to ack_state() instead of
refetching msdata->state in ack_state().
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
--
2.41.0
On Fri, Oct 20, 2023 at 10:43:33PM +0800, Rong Tao wrote:
> From: Rong Tao <[email protected]>
>
> commit b1fc58333575 ("stop_machine: Avoid potential race behaviour")
> solved the race behaviour problem, to better show that race behaviour
> does not exist, pass the 'curstate' directly to ack_state() instead of
> refetching msdata->state in ack_state().
>
I'd prefer if we make this:
| stop_machine: pass curstate to ack_state()
|
| 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.
With that wording:
Acked-by: Mark Rutland <[email protected]>
Mark.
> 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
> --
> 2.41.0
>
Thanks for your advice, Mark.
Your commit information is clearer and easier to understand, I will use it
in the next patch version, thank you.
Rong Tao
On 10/24/23 6:46 PM, Mark Rutland wrote:
> On Fri, Oct 20, 2023 at 10:43:33PM +0800, Rong Tao wrote:
>> From: Rong Tao <[email protected]>
>>
>> commit b1fc58333575 ("stop_machine: Avoid potential race behaviour")
>> solved the race behaviour problem, to better show that race behaviour
>> does not exist, pass the 'curstate' directly to ack_state() instead of
>> refetching msdata->state in ack_state().
>>
> I'd prefer if we make this:
>
> | stop_machine: pass curstate to ack_state()
> |
> | 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.
>
> With that wording:
>
> Acked-by: Mark Rutland <[email protected]>
>
> Mark.
Hi, Mark, I just submit a single patch [0] individually, not as a patchset.
please review. thank you.
Rong Tao
[0]
https://lore.kernel.org/lkml/[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
>> --
>> 2.41.0
>>