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

[Boqun Feng <boqun@kernel.org>]

On Wed, Feb 11, 2026 at 01:19:23PM -0500, Tamir Duberstein wrote:
> On Wed, Feb 11, 2026 at 10:00 AM Boqun Feng <boqun@kernel.org> wrote:
> >
> > On Wed, Feb 11, 2026 at 09:32:36AM -0500, Liam R. Howlett wrote:
> > > * Alice Ryhl <aliceryhl@google.com> [260210 16:34]:
> > > > On Tue, Feb 10, 2026 at 10:23 PM Tamir Duberstein <tamird@kernel.org> wrote:
> > > > >
> > > > > On Tue, Feb 10, 2026 at 12:59 PM Liam R. Howlett
> > > > > <Liam.Howlett@oracle.com> wrote:
> > > > > > Is this a temporary limitation?
> > > > >
> > > > > Maybe? I don't think RfL has good abstractions for RCU yet. For
> > > > > example, exposing load directly on the xarray using xa_load would
> > > > > require a way to guarantee that the returned pointer's target isn't
> >
> > Well, if we only return a pointer, we don't need to guarantee that,
> > right? Because it's up to the user to provide that guarantee. So we
> > could have XArray::load() (not Guard::load()) that just calls xa_load().
> > Also see below.
> >
> > > > > being concurrently mutated (e.g. under the xarray lock). I'm not aware
> > > > > of anyone asking for this, though.
> > > >
> > > > It's relatively easy to add an rcu-backed load using the RCU
> > > > abstractions we have today. I already shared an RFC containing such a
> > > > method for the maple tree, and it would not be much different for
> > > > xarray.
> > > > https://lore.kernel.org/all/20260116-rcu-box-v1-0-38ebfbcd53f0@google.com/
> >
> > I need to point out a difference between xas_load() and Alice's usage
> > (also what Tamir mentioned above) there, what Alice needs (at least from
> > her patchset) is the existence of the object is protected by RCU, i.e.
> > if there is someone else dropping the object, a RCU read lock would
> > still guarantee the access to the object is valid.
> >
> > However, the internal RCU usage of both xarray and maple tree is to
> > protect the *internal* data structure if I'm not missing anything, i.e.
> > an writer may change the array or the tree while a reader is reading,
> > the internal structure itself is still consistent and valid. But the
> > nothing guarantees the object you read is still valid. For example, you
> > can have an xa_erase() racing with an xa_load():
> >
> >         <writer>                        <reader>
> >         ptr = xa_erase(xa, idx);
> >                                         ptr = xa_load(xa, idx);
> >         reclaim(ptr);
> >                                         use(ptr); // <- object may be gone
> >
> > the users of xarray needs to use other mechanism to guarantee the
> > existence of the object.
> >
> > In Alice's case, she in fact used an RCU read side critical section with
> > a larger scope to protect the object as well, which is definitely nice
> > to have, but not only way of using maple/xarray.
> >
> > > >
> > >
> > > It would probably be worth having two loads then, one that does
> > > rcu_read_lock()/unlock() and one for writer/advanced users like we have
> > > on the C side of things.
> > >
> >
> > Agreed. But we may need more ;-)
> >
> > Here IIUC that Andreas does is adding a `load()` for `Guard` of
> > `XArray`, which is the load for a writer and most certainly you won't
> > need to take an RCU read lock for that. The load of a reader can be
> > added as I suggested above (similar as your "rcu_read_lock()/unlock()"
> > suggestion above), but no object existence guarantee. We likely
> > need a third API that can provide the object existence similar to what
> > Alice had in maple tree.
> >
> > > Or at least name the load() function to indicate which is implemented
> > > today?
> > >
> >
> > It's a namespace thing ;-) , the function in this patch is
> > kernel::xarray::Guard::load(), and as I suggest here
> > kernel::xarray::XArray::load() should be the same as xa_load().
> 
> Just to clarify: `kernel::xarray::XArray::load()` does not currently
> exist and no one has yet asked for it.
> 

Yeah ;-) I meant to say we can add it if there is a user and we can use
xa_load() to implement that.

Regards,
Boqun

> >
> > Regards,
> > Boqun
> >
> > > At least on the maple tree side, we have both interfaces and users for
> > > both.  I just found the change to remove the rcu safety odd because I
> > > assumed both are needed.
> > >
> > > Thanks,
> > > Liam
> >