Re: [PATCH 2/3] rust: sync: add SetOnce::try_get_or_populate()

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Jul 29, 2026 at 11:05:55PM +0900, Alexandre Courbot wrote:
> On Wed Jul 22, 2026 at 6:16 PM JST, Alice Ryhl wrote:
> > +    where
> > +        B: lock::Backend,
> > +        F: FnOnce() -> Result<T, E>,
> > +    {
> > +        if let Some(value) = self.as_ref() {
> > +            return Ok(value);
> > +        }
> > +
> > +        let mut to_insert = f()?;
> 
> This means that `f` can run more than once for a given `SetOnce`, which
> can lead to problems depending on `f`'s' side-effects.
> 
> In the GEM shmem case, we would create a second `SGTableMap`, and since
> `SGTableMap` assumes it is the sole owner, the last instance to drop
> would create a use-after-free.
> 
> Now this sounds more like a problem with `SGTableMap`, but if we cannot
> avoid calling `f` at least twice then I think it would help if this was
> documented.

Hmm ... this is the behavior I want in Binder. Actually creating the
value is an allocation, but my lock is a spinlock so I cannot invoke f()
under the lock. This means that each caller will make their own
allocation, and then we throw away any extras if there are concurrent
callers.

If GEM shmem wants f() invoked under the lock, then that's just a
different operation than the one Binder wants.

Do you think we should add both?

Alice