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Howlett" , Alice Ryhl , Andreas Hindborg , Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Trevor Gross , Danilo Krummrich , Lorenzo Stoakes , Vlastimil Babka , Andrew Morton , Christoph Lameter , David Rientjes , Roman Gushchin , Harry Yoo , Daniel Gomez , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v3 05/12] rust: xarray: use `xas_load` instead of `xa_load` in `Guard::load` Message-ID: References: <20260209-xarray-entry-send-v3-0-f777c65b8ae2@kernel.org> <20260209-xarray-entry-send-v3-5-f777c65b8ae2@kernel.org> <6sd4numrxigibwynr2lfmtdj5uabggrx2l57igtjbnq4uwddrm@wipmwcmqbkjn> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Feb 11, 2026 at 01:19:23PM -0500, Tamir Duberstein wrote: > On Wed, Feb 11, 2026 at 10:00 AM Boqun Feng wrote: > > > > On Wed, Feb 11, 2026 at 09:32:36AM -0500, Liam R. Howlett wrote: > > > * Alice Ryhl [260210 16:34]: > > > > On Tue, Feb 10, 2026 at 10:23 PM Tamir Duberstein wrote: > > > > > > > > > > On Tue, Feb 10, 2026 at 12:59 PM Liam R. Howlett > > > > > 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(): > > > > > > 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 > >