Re: [PATCH v6 4/9] rust: sync: atomic: Add generic atomics

["Benno Lossin" <lossin@kernel.org>]

On Fri Jul 11, 2025 at 3:22 PM CEST, Boqun Feng wrote:
> On Fri, Jul 11, 2025 at 10:03:07AM +0200, Benno Lossin wrote:
> [...]
>> > +
>> > +    /// Returns a pointer to the underlying atomic variable.
>> > +    ///
>> > +    /// Extra safety requirement on using the return pointer: the operations done via the pointer
>> > +    /// cannot cause data races defined by [`LKMM`].
>> 
>> I don't think this is correct. I could create an atomic and then share
>> it with the C side via this function, since I have exclusive access, the
>> writes to this pointer don't need to be atomic.
>> 
>
> that's why it says "the operations done via the pointer cannot cause
> data races .." instead of saying "it must be atomic".

Ah right I misread... But then the safety requirement is redundant? Data
races are already UB...

>> We also don't document additional postconditions like this... If you
>
> Please see how Rust std document their `as_ptr()`:
>
> 	https://doc.rust-lang.org/std/sync/atomic/struct.AtomicI32.html#method.as_ptr
>
> It mentions that "Doing non-atomic reads and writes on the resulting
> integer can be a data race." (although the document is a bit out of
> date, since non-atomic read and atomic read are no longer data race now,
> see [1])

That's very different from the comment you wrote though. It's not an
additional safety requirement, but rather a note to users of the API
that they should be careful with the returned pointer.

> I think we can use the similar document structure here: providing more
> safety requirement on the returning pointers, and...
>
>> really would have to do it like this (which you shouldn't given the
>> example above), you would have to make this function `unsafe`, otherwise
>> there is no way to ensure that people adhere to it (since it isn't part
>> of the safety docs).
>> 
>
> ...since dereferencing pointers is always `unsafe`, users need to avoid
> data races anyway, hence this is just additional information that helps
> reasoning.

I disagree.

As mentioned above, data races are already forbidden for raw pointers.
We should indeed add a note that says that non-atomic operations might
result in data races. But that's very different from adding an
additional safety requirement for using the pointer.

And I don't think that we can add additional safety requirements to
dereferencing a raw pointer without an additional `unsafe` block.

---
Cheers,
Benno