Re: [PATCH v6 2/2] rust_binder: move (e)poll wait queue to Process

[Alice Ryhl <aliceryhl@google.com>]

On Fri, Jul 10, 2026 at 05:30:30PM -0700, Boqun Feng wrote:
> On Tue, Jul 07, 2026 at 10:43:13AM +0000, Alice Ryhl wrote:
> > +        {
> > +            let poll = loop {
> > +                if let Some(poll) = this.poll.as_ref() {
> > +                    break poll;
> > +                }
> > +
> > +                let poll = PollCondVarBox::new(c"Process::poll", kernel::static_lock_class!())?;
> > +                // Reuse our existing lock to synchronize callers initializing.
> > +                let _guard = this.node_refs.lock();
> > +                this.poll.populate(poll);
> > +            };
> 
> Note sure whether this lock is needed? SetOnce::populate() should be
> atomic, i.e. only one populate() would win?
> 
> Also seems we should have a SetOnce::as_ref_or_populate(&self, default:
> T).

I'm taking this lock because I want to ensure that losers only loop
once. The problem is that just because you lost the race in populate(),
it's not guaranteed that as_ref() will return Some on the next
iteration, since the winner of the race may still be busy executing
populate(). Taking the loop avoids this possibility.

With regards to as_ref_or_populate(), I point you to this discussion for
reasons why this is hard:
https://lore.kernel.org/all/aZLZbN5C3wXgt3kL@google.com/

> The rest looks good to me. FWIW,
> 
> Reviewed-by: Boqun Feng <boqun@kernel.org>

Thanks!

Alice