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

[Boqun Feng]

On Mon, May 19, 2025 at 10:36:52PM +0200, Jann Horn wrote:
> 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.

I don't think ordering is safety related. For example, relaxed atomics
are still safe function. I think it's wrong to mark this as unsafe, do
you have an example that you can construct an UB with pure safe code?

Regards,
Boqun

> >
> > 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?