Re: [PATCH 07/10] rust: xarray: add `find_next` and `find_next_mut`

[Andreas Hindborg <a.hindborg@kernel.org>]

Tamir Duberstein <tamird@gmail.com> writes:

> On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> Add methods to find the next element in an XArray starting from a
>> given index. The methods return a tuple containing the index where the
>> element was found and a reference to the element.
>>
>> The implementation uses the XArray state API via `xas_find` to avoid taking
>> the xarray lock that is already held by `Guard`.
>
> Similarly to the commit message introducing the use of `xas_load`,
> this is not correct because `xa_find` takes and release the RCU lock
> only, not the XArray lock.

Right, thanks for pointing that out.

>
>>
>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>> ---
>>  rust/kernel/xarray.rs | 65 +++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 65 insertions(+)
>>
>> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
>> index ca97134ba2bd0..9d4589979fd1d 100644
>> --- a/rust/kernel/xarray.rs
>> +++ b/rust/kernel/xarray.rs
>> @@ -255,6 +255,71 @@ pub fn get_mut(&mut self, index: usize) -> Option<T::BorrowedMut<'_>> {
>>          Some(unsafe { T::borrow_mut(ptr.as_ptr()) })
>>      }
>>
>> +    fn load_next(&self, index: usize) -> Option<(usize, NonNull<c_void>)> {
>> +        let mut state = XArrayState::new(self, index);
>> +        // SAFETY: `state.state` is always valid by the type invariant of
>> +        // `XArrayState` and the caller holds the lock.
>> +        let ptr = unsafe { bindings::xas_find(&raw mut state.state, usize::MAX) };
>> +        NonNull::new(ptr).map(|ptr| (state.state.xa_index, ptr))
>> +    }
>
> Can this be a method on XArrayState? It seems odd to document a remote
> type's invariant here when we could put that justification on the type
> itself.

Good idea, I'll move it.


Best regards,
Andreas Hindborg