Re: [PATCH 0/3] rust: sync: add SetOnce::try_get_or_populate()

[Alexandre Courbot]

On Wed Jul 22, 2026 at 6:16 PM JST, Alice Ryhl wrote:
> The SetOnce::populate() method does not internally synchronize callers
> that fail to populate the value with the successful call. This means
> that naive loops using as_ref() and populate() can lead to spinning on
> the initialization, which is best avoided. Thus, provide a helper that
> avoids this issue using a user-provided lock.
>
> One potential alternative is to change populate() so that the failing
> caller actually does synchronize with the successful call to populate().
> However, this is somewhat tricky:
>
> * There are users of SetOnce that construct it in const context, and we
>   currently don't have the ability to do that for most locks, so we
>   cannot easily add a lock to SetOnce.
> * Just spinning on the atomic is undesirable unless we disable
>   preemption in the success path. If we do disable preemption, then that
>   raises complications for handling the PREEMPT_RT case.
> * It also raises questions about deadlocks if populate() is called from
>   irqs.
>
> By using a user-provided lock, we do not have to worry about these
> issues inside SetOnce.
>
> This series is based on char-misc-next.
>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>

If would be nice if the series could also include a patch updating the
GEM shmem module, which is another potential user of
`try_get_or_populate` - in particular since the current implementation
doesn't seem to be a perfect match yet (see my comments on patch 2) so
it would be an opportunity to refine it.