Re: [PATCH 3/3] rust: block: convert `block::mq` to use `Refcount`

[Andreas Hindborg <a.hindborg@kernel.org>]

Hi Greg,

"Greg KH" <gregkh@linuxfoundation.org> writes:

> On Fri, Oct 04, 2024 at 04:52:24PM +0100, Gary Guo wrote:
>> There is an operation needed by `block::mq`, atomically decreasing
>> refcount from 2 to 0, which is not available through refcount.h, so
>> I exposed `Refcount::as_atomic` which allows accessing the refcount
>> directly.
>
> That's scary, and of course feels wrong on many levels, but:
>
>
>> @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) {
>>      /// C `struct request`. If the operation fails, `this` is returned in the
>>      /// `Err` variant.
>>      fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> {
>> -        // We can race with `TagSet::tag_to_rq`
>> -        if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
>> -            2,
>> -            0,
>> -            Ordering::Relaxed,
>> -            Ordering::Relaxed,
>> -        ) {
>> +        // To hand back the ownership, we need the current refcount to be 2.
>> +        // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce
>> +        // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying
>> +        // atomics directly.
>> +        if this
>> +            .wrapper_ref()
>> +            .refcount()
>> +            .as_atomic()
>> +            .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed)
>> +            .is_err()
>
> Why not just call rust_helper_refcount_set()?  Or is the issue that you
> think you might not be 2 here?  And if you HAVE to be 2, why that magic
> value (i.e. why not just always be 1 and rely on normal
> increment/decrement?)
>
> I know some refcounts are odd in the kernel, but I don't see where the
> block layer is caring about 2 as a refcount anywhere, what am I missing?

It is in the documentation, rendered version available here [1]. Let me
know if it is still unclear, then I guess we need to update the docs.

Also, my session from Recipes has a little bit of discussion regarding
this refcount and it's use [2].

Best regards,
Andreas


[1] https://rust.docs.kernel.org/kernel/block/mq/struct.Request.html#implementation-details
[2] https://youtu.be/1LEvgkhU-t4?si=B1XnJhzCCNnUtRsI&t=1685