linux-kernel - Re: [PATCH v2 6/7] rust: file: add `DeferredFdCloser`

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Date:   Wed, 13 Dec 2023 12:04:29 +0100
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     benno.lossin@...ton.me, a.hindborg@...sung.com,
        alex.gaynor@...il.com, arve@...roid.com, bjorn3_gh@...tonmail.com,
        brauner@...nel.org, cmllamas@...gle.com, dan.j.williams@...el.com,
        dxu@...uu.xyz, gary@...yguo.net, gregkh@...uxfoundation.org,
        joel@...lfernandes.org, keescook@...omium.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maco@...roid.com, ojeda@...nel.org, peterz@...radead.org,
        rust-for-linux@...r.kernel.org, surenb@...gle.com,
        tglx@...utronix.de, tkjos@...roid.com, viro@...iv.linux.org.uk,
        wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH v2 6/7] rust: file: add `DeferredFdCloser`

On Tue, Dec 12, 2023 at 9:57 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Mon, Dec 11, 2023 at 05:25:18PM -0800, Boqun Feng wrote:
> > On Mon, Dec 11, 2023 at 09:41:28AM -0800, Boqun Feng wrote:
> > > On Mon, Dec 11, 2023 at 03:34:40PM +0000, Alice Ryhl wrote:
> > > > Benno Lossin <benno.lossin@...ton.me> writes:
> > > > > On 12/6/23 12:59, Alice Ryhl wrote:
> > > > > > +    /// Schedule a task work that closes the file descriptor when this task returns to userspace.
> > > > > > +    ///
> > > > > > +    /// Fails if this is called from a context where we cannot run work when returning to
> > > > > > +    /// userspace. (E.g., from a kthread.)
> > > > > > +    pub fn close_fd(self, fd: u32) -> Result<(), DeferredFdCloseError> {
> > > > > > +        use bindings::task_work_notify_mode_TWA_RESUME as TWA_RESUME;
> > > > > > +
> > > > > > +        // In this method, we schedule the task work before closing the file. This is because
> > > > > > +        // scheduling a task work is fallible, and we need to know whether it will fail before we
> > > > > > +        // attempt to close the file.
> > > > > > +
> > > > > > +        // SAFETY: Getting a pointer to current is always safe.
> > > > > > +        let current = unsafe { bindings::get_current() };
> > > > > > +
> > > > > > +        // SAFETY: Accessing the `flags` field of `current` is always safe.
> > > > > > +        let is_kthread = (unsafe { (*current).flags } & bindings::PF_KTHREAD) != 0;
> > > > >
> > > > > Since Boqun brought to my attention that we already have a wrapper for
> > > > > `get_current()`, how about you use it here as well?
> > > >
> > > > I can use the wrapper, but it seems simpler to not go through a
> > > > reference when we just need a raw pointer.
> > > >
> > > > Perhaps we should have a safe `Task::current_raw` function that just
> > > > returns a raw pointer? It can still be safe.
> > > >
> > >
> > > I think we can have a `as_ptr` function for `Task`?
> > >
> > >     impl Task {
> > >         pub fn as_ptr(&self) -> *mut bindings::task_struct {
> > >             self.0.get()
> > >         }
> > >     }
> >
> > Forgot mention, yes a ptr->ref->ptr trip may not be ideal, but I think
> > eventually we will have a task work wrapper, in that case maybe
> > Task::as_ptr() is still needed somehow.
> >
>
> After some more thoughts, I agree `Task::current_raw` may be better for
> the current usage, since we can also use it to wrap a
> `current_is_kthread` method like:
>
>     impl Task {
>         pub fn current_is_kthread() -> bool {
>             let current = Self::current_raw();
>
>             unsafe { (*current).flags & bindings::PF_KTHREAD != 0 }
>         }
>     }

I'll introduce a current_raw, then.

Alice