From: Rong Tao <[email protected]>
Replace smp_wmb()+WRITE_ONCE() with smp_store_release() and add comment.
Signed-off-by: Rong Tao <[email protected]>
---
kernel/stop_machine.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 268c2e581698..cdf4a3fe0348 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -183,8 +183,10 @@ static void set_state(struct multi_stop_data *msdata,
{
/* Reset ack counter. */
atomic_set(&msdata->thread_ack, msdata->num_threads);
- smp_wmb();
- WRITE_ONCE(msdata->state, newstate);
+ /* This smp_store_release() pair with READ_ONCE() in multi_cpu_stop().
+ * Avoid potential access multi_stop_data::state race behaviour.
+ */
+ smp_store_release(&msdata->state, newstate);
}
/* Last one to ack a state moves to the next state. */
--
2.41.0
On Fri, Oct 20, 2023 at 10:43:34PM +0800, Rong Tao wrote:
> From: Rong Tao <[email protected]>
>
> Replace smp_wmb()+WRITE_ONCE() with smp_store_release() and add comment.
>
> Signed-off-by: Rong Tao <[email protected]>
> ---
> kernel/stop_machine.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> index 268c2e581698..cdf4a3fe0348 100644
> --- a/kernel/stop_machine.c
> +++ b/kernel/stop_machine.c
> @@ -183,8 +183,10 @@ static void set_state(struct multi_stop_data *msdata,
> {
> /* Reset ack counter. */
> atomic_set(&msdata->thread_ack, msdata->num_threads);
> - smp_wmb();
> - WRITE_ONCE(msdata->state, newstate);
> + /* This smp_store_release() pair with READ_ONCE() in multi_cpu_stop().
> + * Avoid potential access multi_stop_data::state race behaviour.
> + */
> + smp_store_release(&msdata->state, newstate);
This doesn't match coding style:
/*
* Block comments should look like this, with a leading '/*' line
* before the text and a traling '*/' line afterwards.
*/
See https://www.kernel.org/doc/html/v4.10/process/coding-style.html#commenting
I don't think the "Avoid potential access multi_stop_data::state race
behaviour." text is all that helpful, and I think we can drop that.
In general, it's unusual to pair a smp_store_release() with READ_ONCE(), and
for that to work it relies on dependency ordering and/or hazarding on the
reader side (e.g. the atomic_dec_and_test() is ordered after the READ_ONCE()
since it's an RMW and there's a control dependency, but a plain read could be
reordered w.r.t. the READ_ONCE()). So we probably need to explain that if we're
going to comment on that smp_store_release().
Peter, might it be worth replacing the READ_ONCE() with smp_load_acquire() at
the same time? I know it's not strictly necessary given the ordering we have
today, but it would at least be obvious.
Mark.
> }
>
> /* Last one to ack a state moves to the next state. */
> --
> 2.41.0
>
On 10/24/23 7:01 PM, Mark Rutland wrote:
> On Fri, Oct 20, 2023 at 10:43:34PM +0800, Rong Tao wrote:
>> From: Rong Tao <[email protected]>
>>
>> Replace smp_wmb()+WRITE_ONCE() with smp_store_release() and add comment.
>>
>> Signed-off-by: Rong Tao <[email protected]>
>> ---
>> kernel/stop_machine.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
>> index 268c2e581698..cdf4a3fe0348 100644
>> --- a/kernel/stop_machine.c
>> +++ b/kernel/stop_machine.c
>> @@ -183,8 +183,10 @@ static void set_state(struct multi_stop_data *msdata,
>> {
>> /* Reset ack counter. */
>> atomic_set(&msdata->thread_ack, msdata->num_threads);
>> - smp_wmb();
>> - WRITE_ONCE(msdata->state, newstate);
>> + /* This smp_store_release() pair with READ_ONCE() in multi_cpu_stop().
>> + * Avoid potential access multi_stop_data::state race behaviour.
>> + */
>> + smp_store_release(&msdata->state, newstate);
> This doesn't match coding style:
>
> /*
> * Block comments should look like this, with a leading '/*' line
> * before the text and a traling '*/' line afterwards.
> */
>
> See https://www.kernel.org/doc/html/v4.10/process/coding-style.html#commenting
Thanks, Mark, I'll fix the comment in next patch version.
>
> I don't think the "Avoid potential access multi_stop_data::state race
> behaviour." text is all that helpful, and I think we can drop that.
>
> In general, it's unusual to pair a smp_store_release() with READ_ONCE(), and
> for that to work it relies on dependency ordering and/or hazarding on the
> reader side (e.g. the atomic_dec_and_test() is ordered after the READ_ONCE()
> since it's an RMW and there's a control dependency, but a plain read could be
> reordered w.r.t. the READ_ONCE()). So we probably need to explain that if we're
> going to comment on that smp_store_release().
>
> Peter, might it be worth replacing the READ_ONCE() with smp_load_acquire() at
> the same time? I know it's not strictly necessary given the ordering we have
> today, but it would at least be obvious.
After I wait for Peter to reply to this message, I will write a patch
based on Peter's suggestion.
Rong Tao.
>
> Mark.
>
>> }
>>
>> /* Last one to ack a state moves to the next state. */
>> --
>> 2.41.0
>>