Re: [PATCH 1/3] rust: sync: Add abstraction for synchronize_rcu()

[Alice Ryhl <aliceryhl@google.com>]

On Thu, Jun 18, 2026 at 01:40:05PM +0200, Philipp Stanner wrote:
> On Thu, 2026-06-18 at 11:59 +0200, Philipp Stanner wrote:
> > synchronize_rcu() is a frequently used C function which is always safe
> > to be called.
> > 
> > Add a safe abstraction for synchronize_rcu().
> > 
> > Signed-off-by: Philipp Stanner <phasta@kernel.org>
> > ---
> >  rust/kernel/sync/rcu.rs | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
> > index a32bef6e490b..9b9addf8fedf 100644
> > --- a/rust/kernel/sync/rcu.rs
> > +++ b/rust/kernel/sync/rcu.rs
> > @@ -50,3 +50,11 @@ fn drop(&mut self) {
> >  pub fn read_lock() -> Guard {
> >      Guard::new()
> >  }
> > +
> > +/// Wait for one RCU grace period.
> > +///
> > +/// You typically do this to wait for everyone holding a [`Guard`].
> > +pub fn synchronize_rcu() {
> 
> Hm, should it actually be #[inline]?
> 
> In C nowadays usage of inline is discouraged ("compiler knows better").
> Unsure how Rust handles it; is its compiler different?

If you don't mark it #[inline], then Rust is going to generate a
function for the Rust synchronize_rcu() wrapper just in case a module
wants to call it. The module might still decide to inline it, but
because there's the possibility that a module could choose not to inline
it, it will generate a wrapper in the core kernel.

For simple wrappers around C functions, we generally just want the
module to invoke the C function directly, so #[inline] is good here to
avoid the core kernel function.

> Moreover, this would be an opportunity to change the naming convention
> to rcu::synchronize()
> 
> But since Boqun & Alice are pushing for rcu::RcuKBox for the reason
> that it seems desirable to explicitly highlight that that's a special
> Box, I guess we should be consistent with that and also have "rcu" in
> the name here.

I prefer the naming of synchronize_rcu() here.

Alice