Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Feb 02, 2023 at 10:14:06AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 01, 2023 at 03:22:41PM -0800, Boqun Feng wrote:
> > This allows reading the current count of a refcount in an `ArcInner`.
> > 
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> >  rust/helpers.c          | 6 ++++++
> >  rust/kernel/sync/arc.rs | 9 +++++++++
> >  2 files changed, 15 insertions(+)
> > 
> > diff --git a/rust/helpers.c b/rust/helpers.c
> > index 09a4d93f9d62..afc5f1a39fef 100644
> > --- a/rust/helpers.c
> > +++ b/rust/helpers.c
> > @@ -46,6 +46,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
> >  }
> >  EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
> >  
> > +unsigned int rust_helper_refcount_read(refcount_t *r)
> > +{
> > +	return refcount_read(r);
> > +}
> > +EXPORT_SYMBOL_GPL(rust_helper_refcount_read);
> > +
> >  /*
> >   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
> >   * as the Rust `usize` type, so we can use it in contexts where Rust
> > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> > index fc680a4a795c..fbfceaa3096e 100644
> > --- a/rust/kernel/sync/arc.rs
> > +++ b/rust/kernel/sync/arc.rs
> > @@ -127,6 +127,15 @@ struct ArcInner<T: ?Sized> {
> >      data: T,
> >  }
> >  
> > +impl<T: ?Sized> ArcInner<T> {
> > +    /// Returns the current reference count of [`ArcInner`].
> > +    fn count(&self) -> u32 {
> > +        // SAFETY: `self.refcount.get()` is always a valid pointer, and `refcount_read()` is a
> > +        // normal atomic read (i.e. no data race) only requiring on the address is valid.
> > +        unsafe { bindings::refcount_read(self.refcount.get()) }
> > +    }
> > +}
> 
> This is completely unsafe vs concurrency. In order to enable correct
> tracing of refcount manipulations we have the __refcount_*(.oldp) API.
> 

Interesting.

> Admittedly, I have no idea how to sanely wire that up in this thing, but
> it seems odd to me to have this 'safe' language display fundamentally
> unsafe data like this.

Right now the only use of this "data" is for debugging display, the
"count" function is private, so no one outside Arc can mis-use it, in
that sense, I think it's still safe?

Regards,
Boqun