linux-kernel - Re: [PATCH 05/10] rust: xarray: use `xas_load` instead of `xa

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Message-ID: <CAJ-ks9nBRfYpQKkzuM84TWsXSEccUXxxwQNUPxZHZZxrnXUbCQ@mail.gmail.com>
Date: Wed, 7 Jan 2026 15:06:54 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, Daniel Gomez <da.gomez@...nel.org>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/10] rust: xarray: use `xas_load` instead of `xa_load`
 in `Guard::load`

On Wed, Jan 7, 2026 at 2:37 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> Tamir Duberstein <tamird@...il.com> writes:
>
> > On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
> >>
> >> Replace the call to `xa_load` with `xas_load` in `Guard::load`. The
> >> `xa_load` function takes the XArray lock internally, which would cause
> >> a double lock since the `Guard` already holds the lock.
> >
> > This is not correct. `xa_load` takes and releases the RCU lock only,
> > not the XArray lock.
>
> You are right. However, we do not need to take the RCU lock either in
> this case, right?

Seems reasonable, but the language mentioning double locking should be
removed please.

>
> >
> >> The `xas_load`
> >> function operates on XArray state and assumes the lock is already held,
> >> which is the correct API to use when holding a `Guard`.
> >>
> >> This change also removes the `#[expect(dead_code)]` annotation from
> >> `XArrayState` and its constructor, as they are now in use.
> >>
> >> Signed-off-by: Andreas Hindborg <a.hindborg@...nel.org>
> >> ---
> >>  rust/kernel/xarray.rs | 8 ++++----
> >>  1 file changed, 4 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
> >> index f6e610b059625..0f69a523b72bf 100644
> >> --- a/rust/kernel/xarray.rs
> >> +++ b/rust/kernel/xarray.rs
> >> @@ -215,8 +215,10 @@ fn load<F, U>(&self, index: usize, f: F) -> Option<U>
> >>      where
> >>          F: FnOnce(NonNull<c_void>) -> U,
> >>      {
> >> -        // SAFETY: `self.xa.xa` is always valid by the type invariant.
> >> -        let ptr = unsafe { bindings::xa_load(self.xa.xa.get(), index) };
> >> +        let mut state = XArrayState::new(self, index);
> >> +        // SAFETY: `state.state` is always valid by the type invariant of
> >> +        // `XArrayState`.
> >> +        let ptr = unsafe { bindings::xas_load(&raw mut state.state) };
> >
> > Can this be a method on XArrayState?
>
> OK.
>
>
> Best regards,
> Andreas Hindborg
>
>