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

[Andreas Hindborg]

Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:

> "Gary Guo" <gary@xxxxxxxxxxx> writes:
>
>> On Sat, 5 Oct 2024 13:59:44 +0200
>> Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>>
>>> On Sat, Oct 5, 2024 at 11:49 AM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>>> >
>>> > Hi Greg,
>>> >
>>> > "Greg KH" <gregkh@xxxxxxxxxxxxxxxxxxx> 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
>>>
>>> So it sounds like there is one refcount from the C side, and some
>>> number of references from the Rust side. The function checks whether
>>> there's only one Rust reference left, and if so, takes ownership of
>>> the value, correct?
>>>
>>> In that case, the CAS should have an acquire ordering to synchronize
>>> with dropping the refcount 3->2 on another thread. Otherwise, you
>>> might have a data race with the operations that happened just before
>>> the 3->2 refcount drop.
>>>
>>> Alice
>>
>> The code as is is fine since there's no data protected in
>> `RequestDataWrapper` yet (in fact it's not even generic yet). I know
>> Andreas does want to introduce driver-specific data into that, so in
>> the long term the acquire would be necessary.
>>
>> Andreas, please let me know if you want me to make the change now, or
>> you'd rather change the ordering when you introduce data to
>> `RequestDataWrapper`.
>
> I guess we will have said data dependencies when we are going to run
> drop for fields in the private data area. Thanks for pointing that out.
> I will update the ordering when I submit that patch.
>
> As I mentioned before, I would rather we do not apply this patch before
> we get a way to inline helpers.

As discussed offline, the code that suffers the performance regression
is downstream, and since this change seems to be important, I can apply
the helper LTO patch downstream as well.

Since the plan for the downstream code _is_ to move upstream, I really
hope to see the helper LTO patch upstream, so we don't get a performance
regression because of these refcounts.

If we cannot figure out a way to get the LTO patches (or an alternative
solution) upstream, we can always revert back to a more performant
solution in block.


BR Andreas