Date: Tue, 04 Jun 2024 11:59:49 +0200
From: Andreas Hindborg <nmi@...aspace.dk>
To: Benno Lossin <benno.lossin@...ton.me>
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Subject: Re: [PATCH v4 1/3] rust: block: introduce `kernel::block::mq` module

Benno Lossin <benno.lossin@...ton.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.
- 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.

[...]

>>>> +    /// 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`.


BR Andreas

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