The new SAFETY comment style is taken from existing comments in `deref`
and `drop.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 rust/kernel/sync/arc.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index af383bcd003e1122ebe1b62a49fe40279458e379..9adea755a5ad1a7b03f7fc30a7abc76c1f966c6c 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
         // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { bindings::refcount_inc(refcount) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }

-- 
2.47.0

On Mon, Nov 4, 2024 at 10:20 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> The new SAFETY comment style is taken from existing comments in `deref`
> and `drop.
>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index af383bcd003e1122ebe1b62a49fe40279458e379..9adea755a5ad1a7b03f7fc30a7abc76c1f966c6c 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
>
>  impl<T: ?Sized> Clone for Arc<T> {
>      fn clone(&self) -> Self {
> +        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> +        // safe to dereference it.
> +        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();

I would normally prefer to avoid creating a reference to the entire
ArcInner, but in this particular case it is okay due to the specifics
of how Arc works.

Alice

On Fri, Nov 8, 2024 at 6:38 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > index af383bcd003e1122ebe1b62a49fe40279458e379..9adea755a5ad1a7b03f7fc30a7abc76c1f966c6c 100644
> > --- a/rust/kernel/sync/arc.rs
> > +++ b/rust/kernel/sync/arc.rs
> > @@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
> >
> >  impl<T: ?Sized> Clone for Arc<T> {
> >      fn clone(&self) -> Self {
> > +        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> > +        // safe to dereference it.
> > +        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
>
> I would normally prefer to avoid creating a reference to the entire
> ArcInner, but in this particular case it is okay due to the specifics
> of how Arc works.

Note that this particular line appears also in the Drop impl just
below. That said, can you help me understand the concern with creating
a reference to the entire ArcInner?

On Fri, Nov 8, 2024 at 1:30 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> On Fri, Nov 8, 2024 at 6:38 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > > index af383bcd003e1122ebe1b62a49fe40279458e379..9adea755a5ad1a7b03f7fc30a7abc76c1f966c6c 100644
> > > --- a/rust/kernel/sync/arc.rs
> > > +++ b/rust/kernel/sync/arc.rs
> > > @@ -377,10 +377,14 @@ fn as_ref(&self) -> &T {
> > >
> > >  impl<T: ?Sized> Clone for Arc<T> {
> > >      fn clone(&self) -> Self {
> > > +        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> > > +        // safe to dereference it.
> > > +        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
> >
> > I would normally prefer to avoid creating a reference to the entire
> > ArcInner, but in this particular case it is okay due to the specifics
> > of how Arc works.
>
> Note that this particular line appears also in the Drop impl just
> below. That said, can you help me understand the concern with creating
> a reference to the entire ArcInner?

Creating a shared reference to the entire ArcInner is an assertion
that no &mut exists to any part of the ArcInner, even though you only
access the refcount field. In this particular case, asserting shared
access to the `data` field is not a problem due to how Arc works, so
it's okay, but the pattern is problematic in other cases.

You could write `unsafe { (*self.ptr.as_ptr()).refcount.get() }` to
avoid the as_ref call.

Alice

On Fri, Nov 8, 2024 at 7:37 AM Alice Ryhl <aliceryhl@google.com> wrote:
> You could write `unsafe { (*self.ptr.as_ptr()).refcount.get() }` to
> avoid the as_ref call.

Doesn't `(*self.ptr.as_ptr())` have the same problem?

On Fri, Nov 8, 2024 at 1:41 PM Tamir Duberstein <tamird@gmail.com> wrote:
>
> On Fri, Nov 8, 2024 at 7:37 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > You could write `unsafe { (*self.ptr.as_ptr()).refcount.get() }` to
> > avoid the as_ref call.
>
> Doesn't `(*self.ptr.as_ptr())` have the same problem?

It's not quite the same. Both `*ptr` and `(*ptr).field` are place
expressions which do not inherently involve the creation of a
reference in the way that `NonNull::as_ref` does.

Alice

On Fri, Nov 8, 2024 at 7:47 AM Alice Ryhl <aliceryhl@google.com> wrote:
>
> On Fri, Nov 8, 2024 at 1:41 PM Tamir Duberstein <tamird@gmail.com> wrote:
> >
> > On Fri, Nov 8, 2024 at 7:37 AM Alice Ryhl <aliceryhl@google.com> wrote:
> > > You could write `unsafe { (*self.ptr.as_ptr()).refcount.get() }` to
> > > avoid the as_ref call.
> >
> > Doesn't `(*self.ptr.as_ptr())` have the same problem?
>
> It's not quite the same. Both `*ptr` and `(*ptr).field` are place
> expressions which do not inherently involve the creation of a
> reference in the way that `NonNull::as_ref` does.

Thanks for explaining!
Tamir