Re: [PATCH v4 1/3] rust: block: introduce `kernel::block::mq` module

Benno Lossin
2024-06-10T20:07:44Z

On 04.06.24 11:59, Andreas Hindborg wrote:
> Benno Lossin <benno.lossin@proton.me> writes:
>
> [...]
>
>>>>> +impl<T: Operations> OperationsVTable<T> {
>>>>> + /// This function is called by the C kernel. A pointer to this function is
>>>>> + /// installed in the `blk_mq_ops` vtable for the driver.
>>>>> + ///
>>>>> + /// # Safety
>>>>> + ///
>>>>> + /// - The caller of this function must ensure `bd` is valid
>>>>> + /// and initialized. The pointees must outlive this function.
>>>>
>>>> Until when do the pointees have to be alive? "must outlive this
>>>> function" could also be the case if the pointees die immediately after
>>>> this function returns.
>>>
>>> It should not be plural. What I intended to communicate is that what
>>> `bd` points to must be valid for read for the duration of the function
>>> call. I think that is what "The pointee must outlive this function"
>>> states? Although when we talk about lifetime of an object pointed to by
>>> a pointer, I am not sure about the correct way to word this. Do we talk
>>> about the lifetime of the pointer or the lifetime of the pointed to
>>> object (the pointee). We should not use the same wording for the pointer
>>> and the pointee.
>>>
>>> How about:
>>>
>>> /// - The caller of this function must ensure that the pointee of `bd` is
>>> /// valid for read for the duration of this function.
>>
>> But this is not enough for it to be sound, right? You create an `ARef`
>> from `bd.rq`, which potentially lives forever. You somehow need to
>> require that the pointer `bd` stays valid for reads and (synchronized)
>> writes until the request is ended (probably via `blk_mq_end_request`).
>
> The statement does not say anything about `*((*bd).rq)`. `*bd` needs to
> be valid only for the duration of the function. It carries a pointer to
> a `struct request` in the `rq` field. The pointee of that pointer must
> be exclusively owned by the driver until the request is done.
>
> Maybe like this:
>
> # Safety
>
> - The caller of this function must ensure that the pointee of `bd` is
> valid for read for the duration of this function.

"valid for reads"

> - This function must be called for an initialized and live `hctx`. That
> is, `Self::init_hctx_callback` was called and
> `Self::exit_hctx_callback()` was not yet called.
> - `(*bd).rq` must point to an initialized and live `bindings:request`.
> That is, `Self::init_request_callback` was called but
> `Self::exit_request_callback` was not yet called for the request.
> - `(*bd).rq` must be owned by the driver. That is, the block layer must
> promise to not access the request until the driver calls
> `bindings::blk_mq_end_request` for the request.

Sounds good!

> [...]
>
>>>>> + /// This function is called by the C kernel. A pointer to this function is
>>>>> + /// installed in the `blk_mq_ops` vtable for the driver.
>>>>> + ///
>>>>> + /// # Safety
>>>>> + ///
>>>>> + /// This function may only be called by blk-mq C infrastructure. `set` must
>>
>> `set` doesn't exist (`_set` does), you are also not using this
>> requirement.
>
> Would be nice if there was a way in `rustdoc` no name arguments
> explicitly.
>
>>
>>>>> + /// point to an initialized `TagSet<T>`.
>>>>> + unsafe extern "C" fn init_request_callback(
>>>>> + _set: *mut bindings::blk_mq_tag_set,
>>>>> + rq: *mut bindings::request,
>>>>> + _hctx_idx: core::ffi::c_uint,
>>>>> + _numa_node: core::ffi::c_uint,
>>>>> + ) -> core::ffi::c_int {
>>>>> + from_result(|| {
>>>>> + // SAFETY: The `blk_mq_tag_set` invariants guarantee that all
>>>>> + // requests are allocated with extra memory for the request data.
>>>>
>>>> What guarantees that the right amount of memory has been allocated?
>>>> AFAIU that is guaranteed by the `TagSet` (but there is no invariant).
>>>
>>> It is by C API contract. `TagSet`::try_new` (now `new`) writes
>>> `cmd_size` into the `struct blk_mq_tag_set`. That is picked up by
>>> `blk_mq_alloc_tag_set` to allocate the right amount of space for each request.
>>>
>>> The invariant here is on the C type. Perhaps the wording is wrong. I am
>>> not exactly sure how to express this. How about this:
>>>
>>> // SAFETY: We instructed `blk_mq_alloc_tag_set` to allocate requests
>>> // with extra memory for the request data when we called it in
>>> // `TagSet::new`.
>>
>> I think you need a safety requirement on the function: `rq` points to a
>> valid `Request`. Then you could just use `Request::wrapper_ptr` instead
>> of the line below.
>
> I cannot require `rq` to point to a valid `Request`, because that would
> require the private data area to already be initialized as a valid
> `RequestDataWrapper`. Using the `wrapper_ptr` is good 👍. How is this:
>
>
> /// # Safety
> ///
> /// - This function may only be called by blk-mq C infrastructure.
> /// - `_set` must point to an initialized `TagSet<T>`.
> /// - `rq` must point to an initialized `bindings::request`.
> /// - The allocation pointed to by `rq` must be at the size of `Request`
> /// plus the size of `RequestDataWrapper`.

Also sounds good to me.

---
Cheers,
Benno