Re: [PATCH v8 3/8] rust: file: add Rust abstraction for `struct file`

[Boqun Feng]

On Wed, Aug 07, 2024 at 10:50:32AM +0200, Alice Ryhl wrote:
> On Tue, Aug 6, 2024 at 9:30 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 06, 2024 at 10:48:11AM +0200, Alice Ryhl wrote:
> > [...]
> > > > > +    /// Returns the flags associated with the file.
> > > > > +    ///
> > > > > +    /// The flags are a combination of the constants in [`flags`].
> > > > > +    #[inline]
> > > > > +    pub fn flags(&self) -> u32 {
> > > > > +        // This `read_volatile` is intended to correspond to a READ_ONCE call.
> > > > > +        //
> > > > > +        // SAFETY: The file is valid because the shared reference guarantees a nonzero refcount.
> > > > > +        //
> > > > > +        // FIXME(read_once): Replace with `read_once` when available on the Rust side.
> > > >
> > > > Do you know the status of this?
> > >
> > > It's still unavailable.
> > >
> >
> > I think with our own Atomic API, we can just use atomic_read() here:
> > yes, I know that to make this is not a UB, we need the C side to also do
> > atomic write on this `f_flags`, however, my reading of C code seems to
> > suggest that FS relies on writes to this field is atomic, therefore
> > unless someone is willing to convert all writes to `f_flags` in C into
> > a WRITE_ONCE(), nothing more we can do on Rust side. So using
> > atomic_read() is the correct thing to begin with.
> 
> Huh? The C side uses atomic reads for this?
> 

Well, READ_ONCE(->f_flags) is atomic, so I thought you want to use
atomic here. However, after a quick look of `->f_flags` accesses, I find
out they should be protected by `->f_lock` (a few cases rely on
data race accesses, see p4_fd_open()), so I think what you should really
do here is the similar: make sure Rust code only accesses `->f_flags`
if `->f_lock` is held. Unless that's not the case for binder?

Regards,
Boqun

> Alice