Re: [PATCH RFC 0/2] rcu box container for Rust + maple tree load_rcu

["Gary Guo" <gary@garyguo.net>]

On Sat Jan 17, 2026 at 3:39 PM GMT, Boqun Feng wrote:
> On Sat, Jan 17, 2026 at 10:05:18PM +0800, Boqun Feng wrote:
>> On Sat, Jan 17, 2026 at 01:29:25PM +0000, Alice Ryhl wrote:
>> > On Sat, Jan 17, 2026 at 09:11:49PM +0800, Boqun Feng wrote:
>> > > In Rcu + RcuOld design, RcuBox is just a Box
>> > > because these two pointer types handle reclaim + accesses.
>> > 
>> > How would that work? Dropping my RcuBox<_> must use kfree_rcu() or
>> > synchronize_rcu() or it is unsound. So it can't just be a Box.
>> > 
>> 
>> RcuOld<P: ForeignOwnable> will call synchronize_rcu() before dropping
>> `P`. And I think we can have an asynchronous drop pointer type like
>> `RcuOld<P, Async>` that requires `P` is a type like `Box<HasRcuHead>`,
>
> One blocker on that is Drop impls cannot be specialized. We probably
> need a different pointer type like RcuOldAsync...

I won't be too worried on this apsect, you could just branch on a const in the
drop function.

Alternatively, we can have a method like this

impl<T: HasRcuHead> RcuOld<Box<T>> {
    fn drop_async(self) {
        // do call_rcu
    }
}

Best,
Gary