Re: [PATCH v2 3/4] rust: devres: get rid of Devres' inner Arc

Danilo Krummrich
2025-06-24T16:15:43Z

On Tue, Jun 24, 2025 at 08:46:53AM -0700, Boqun Feng wrote:
> On Tue, Jun 24, 2025 at 05:18:23PM +0200, Danilo Krummrich wrote:
> > On Sun, Jun 22, 2025 at 06:54:07PM -0700, Boqun Feng wrote:
> > > I think you also need to mention that `inner` only remains valid until
> > > `inner.devm.complete_all()` unblocks `Devres::drop()`, because after
> > > `Devres::drop()`'s `devm.wait_for_completion()` returns, `inner` may be
> > > dropped or freed.
> >
> > I think of it the other way around: The invariant guarantees that `inner` is
> > *always* valid.
> >
> > The the `drop_in_place(inner)` call has to justify that it upholds this
> > invariant, by ensuring that at the time it is called no other code that accesses
> > `inner` can ever run.
> >
> > Defining it the other way around would make the `inner()` accessor unsafe.
>
> Maybe I wasn't clear enough, I meant in the following function:
>
> unsafe extern "C" fn devres_callback(ptr: *mut kernel::ffi::c_void) {
> - let ptr = ptr as *mut DevresInner<T>;
> - // Devres owned this memory; now that we received the callback, drop the `Arc` and hence the
> - // reference.
> - // SAFETY: Safe, since we leaked an `Arc` reference to devm_add_action() in
> - // `DevresInner::new`.
> - let inner = unsafe { Arc::from_raw(ptr) };
> + // SAFETY: In `Self::new` we've passed a valid pointer to `Inner` to `devm_add_action()`,
> + // hence `ptr` must be a valid pointer to `Inner`.
> + let inner = unsafe { &*ptr.cast::<Inner<T>>() };
>
> ^ this `inner` was constructed by reborrowing from `ptr`, but it should
> only be used before the following `inner.devm.complete_all()`...

Oh, so you meant adding this to the safety comment. Yes, that makes sense. Maybe
ScopeGuard works too, as you say.