Re: [PATCH v3 05/12] rust: xarray: use `xas_load` instead of `xa_load` in `Guard::load`

[Liam R. Howlett]

* Tamir Duberstein <tamird@xxxxxxxxxx> [260210 19:54]:
> On Tue, Feb 10, 2026 at 10:16 AM Liam R. Howlett
> <Liam.Howlett@xxxxxxxxxx> wrote:
> >
> > * Andreas Hindborg <a.hindborg@xxxxxxxxxx> [260209 14:39]:
> > > Replace the call to `xa_load` with `xas_load` in `Guard::load`. The
> > > `xa_load` function takes the RCU lock internally, which we do not need,
> > > since the `Guard` already holds an exclusive lock on the `XArray`. The
> > > `xas_load` function operates on `xa_state` and assumes the required locks
> > > are already held.
> > >
> > > This change also removes the `#[expect(dead_code)]` annotation from
> > > `XArrayState` and its constructor, as they are now in use.
> >
> > I don't understand the locking here.
> >
> > You are saying that, since you hold the xarray write lock, you won't be
> > taking the rcu read lock, but then you change the api of load?  That
> > seems wrong to me.
> 
> This patch doesn't change the API of load. Andreas is saying that the
> type system already requires the caller to hold the xarray spin lock
> when load is called, meaning acquiring the RCU lock isn't necessary.

What I mean is that the API can no longer be called when holding the RCU
read lock.  You seem to imply this is already the case though.

> 
> >
> > Any readers of the api that calls load will now need to hold the rcu
> > read lock externally.  If you're doing this, then you should indicate
> > that is necessary in the function name, like the C side does.  Otherwise
> > you are limiting the users to the advanced API, aren't you?
> 
> The existing API already requires users to hold the xarray lock.
> 
> >
> > Or are you saying that xarray can only be used if you hold the exclusive
> > lock, which is now a read and write lock?
> 
> Yes - except for the word "now"; I'm not sure what you mean by it.

I'm trying to understand the locking on the rust side.

I think you answered it by telling me that all readers and writers use
the spinlock.

Is this a temporary limitation?

Thanks,
Liam