Re: [PATCH v8 3/5] rust: add bitmap API.

[Jann Horn]

On Mon, May 19, 2025 at 10:08 PM Burak Emir <bqe@xxxxxxxxxx> wrote:
> On Mon, May 19, 2025 at 9:01 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
> > On Mon, May 19, 2025 at 6:24 PM Burak Emir <bqe@xxxxxxxxxx> wrote:
> > > +    /// Set bit with index `index`, atomically.
> > > +    ///
> > > +    /// ATTENTION: The naming convention differs from C, where the corresponding
> > > +    /// function is called `set_bit`.
> > > +    ///
> > > +    /// # Safety
> > > +    ///
> > > +    /// This is a relaxed atomic operation (no implied memory barriers, no
> > > +    /// ordering guarantees). The caller must ensure that this is safe, as
> > > +    /// the compiler cannot prevent code with an exclusive reference from
> > > +    /// calling atomic operations.
> >
> > How can atomic operations through an exclusive reference be unsafe?
> > You can't have a data race between two atomic operations, and an
> > exclusive reference should anyway prevent any concurrency, right?
>
> The atomic operations take a &self (shared reference).
>
> The patch is missing the implementation of Sync for now. With that,
> one would get concurrent write access through shared references.
>
> The "unsafe" here should serve as reminder to argue why it is ok to
> not have any ordering guarantees.
>
> The last sentence is supposed to say: when you have a &mut bitmap, you
> can reborrow it as &bitmap, and then happily call this atomic op.
> Even though it is unnecessary.

But using an atomic op when you have a &mut reference is not a safety
issue, right? You wrote a comment about behavior with exclusive
references in the "# Safety" comment block. If that's not supposed to
be a safety problem, this should probably not be in the "# Safety"
section?