Re: [PATCH v12 2/3] rust: add dma coherent allocator abstraction.

[Andreas Hindborg]

"Alice Ryhl" <aliceryhl@xxxxxxxxxx> writes:

> On Mon, Mar 3, 2025 at 2:00 PM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>>
>> "Daniel Almeida" <daniel.almeida@xxxxxxxxxxxxx> writes:
>>
>> > Hi Benno,
>> >
>>
>> [...]
>>
>> >>> +    /// Writes data to the region starting from `offset`. `offset` is in units of `T`, not the
>> >>> +    /// number of bytes.
>> >>> +    ///
>> >>> +    /// # Examples
>> >>> +    ///
>> >>> +    /// ```
>> >>> +    /// # fn test(alloc: &mut kernel::dma::CoherentAllocation<u8>) -> Result {
>> >>> +    /// let somedata: [u8; 4] = [0xf; 4];
>> >>> +    /// let buf: &[u8] = &somedata;
>> >>> +    /// alloc.write(buf, 0)?;
>> >>> +    /// # Ok::<(), Error>(()) }
>> >>> +    /// ```
>> >>> +    pub fn write(&self, src: &[T], offset: usize) -> Result {
>> >>> +        let end = offset.checked_add(src.len()).ok_or(EOVERFLOW)?;
>> >>> +        if end >= self.count {
>> >>> +            return Err(EINVAL);
>> >>> +        }
>> >>> +        // SAFETY:
>> >>> +        // - The pointer is valid due to type invariant on `CoherentAllocation`
>> >>> +        // and we've just checked that the range and index is within bounds.
>> >>> +        // - `offset` can't overflow since it is smaller than `selfcount` and we've checked
>> >>> +        // that `self.count` won't overflow early in the constructor.
>> >>> +        unsafe {
>> >>> +            core::ptr::copy_nonoverlapping(src.as_ptr(), self.cpu_addr.add(offset), src.len())
>> >>
>> >> Why are there no concurrent write or read operations on `cpu_addr`?
>> >
>> > Sorry, can you rephrase this question?
>>
>> This write is suffering the same complications as discussed here [1].
>> There are multiple issues with this implementation.
>>
>> 1) `write` takes a shared reference and thus may be called concurrently.
>> There is no synchronization, so `copy_nonoverlapping` could be called
>> concurrently on the same address. The safety requirements for
>> `copy_nonoverlapping` state that the destination must be valid for
>> write. Alice claims in [1] that any memory area that experience data
>> races are not valid for writes. So the safety requirement of
>> `copy_nonoverlapping` is violated and this call is potential UB.
>>
>> 2) The destination of this write is DMA memory. It could be concurrently
>> modified by hardware, leading to the same issues as 1). Thus the
>> function cannot be safe if we cannot guarantee hardware will not write
>> to the region while this function is executing.
>>
>> Now, I don't think that these _should_ be issues, but according to our
>> Rust language experts they _are_.
>>
>> I really think that copying data through a raw pointer to or from a
>> place that experiences data races, should _not_ be UB if the data is not
>> interpreted in any way, other than moving it.
>>
>>
>> Best regards,
>> Andreas Hindborg
>
> We need to make progress on this series, and it's starting to get late
> in the cycle. I suggest we:

There is always another cycle.

>
> 1. Delete as_slice, as_slice_mut, write, and skip_drop.
> 2. Change field_read/field_write to use a volatile read/write.

Volatile reads/writes that race are OK?

>
> This will let us make progress now and sidestep this discussion. The
> deleted methods can happen in a follow-up.

`item_from_index`, the `dma_read` and `dma_write` macros as well, I would think?

>
> Similarly for the dma mask methods, let's either drop them to a
> follow-up patch or just put them anywhere and move them later.

Sure.


Best regards,
Andreas Hindborg