On Fri, Nov 22, 2024 at 07:48:16PM +0100, Alice Ryhl wrote:
> On Fri, Nov 22, 2024 at 7:03 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> >
> > On Fri, Nov 22, 2024 at 03:40:33PM +0000, Alice Ryhl wrote:
> > > +/// Represents a [`Task`] obtained from the `current` global.
> > > +///
> > > +/// This type exists to provide more efficient operations that are only valid on the current task.
> > > +/// For example, to retrieve the pid-namespace of a task, you must use rcu protection unless it is
> > > +/// the current task.
> > > +///
> > > +/// # Invariants
> > > +///
> > > +/// Must be equal to `current` of some thread that is currently running somewhere.
> > > +pub struct CurrentTask(Task);
> > > +
> >
> > I think you need to make `CurrentTask` `!Sync`, right? Otherwise, other
> > threads can access the shared reference of a `CurrentTask` and the ->mm
> > field. I'm thinking if we have a scoped thread/workqueue support in the
> > future:
> >
> > let task = current!();
> > Workqueue::scoped(|s| {
> > s.spawn(|| {
> > let mm = task.mm();
> > // do something with the mm
> > });
> > });
>
> I don't think this is a problem? As long as a thread exists somewhere
> with `current` being equal to the task, we should be fine?
>
I think I had a misunderstanding on what you meant by "operations
that are only valid on the current task", you mean these operations can
be run by other threads, but it has to be *on* a task_struct that's
"currently running", right? BTW, you probably want to reword a bit,
because the "current" task may be blocked, so technically it's not
"running".
Basically, the operations that `CurrentTask` have are the methods that
are safe to call (even on a different thread) for the "current" task, as
long as it exists (not dead or exited). In that definition, not being
`Sync` is fine.
But I have to admit I'm a bit worried that people may be confused, and
add new methods that can be only run by the current thread in the
future.
> > > +impl CurrentTask {
> > > + /// Access the address space of this task.
> > > + ///
> > > + /// To increment the refcount of the referenced `mm`, you can use `ARef::from`.
> > > + #[inline]
> > > + pub fn mm(&self) -> Option<&MmWithUser> {
> >
> > Hmm... similar issue, `MmWithUser` is `Sync`.
>
> What is the problem with that?
>
It should be no problem under your definition of `CurrentTask`.
Regards,
Boqun
> > > + let mm = unsafe { (*self.as_ptr()).mm };
> > > +
> > > + if mm.is_null() {
> > > + None
> > > + } else {
> > > + // SAFETY: If `current->mm` is non-null, then it references a valid mm with a non-zero
> > > + // value of `mm_users`. The returned `&MmWithUser` borrows from `CurrentTask`, so the
> > > + // `&MmWithUser` cannot escape the current task, meaning `mm_users` can't reach zero
> > > + // while the reference is still live.
> >
> > Regards,
> > Boqun
> >
> > > + Some(unsafe { MmWithUser::from_raw(mm) })
> > > + }
> > > + }
> > > +}
> > > +
> > > // SAFETY: The type invariants guarantee that `Task` is always refcounted.
> > > unsafe impl crate::types::AlwaysRefCounted for Task {
> > > fn inc_ref(&self) {
> > >
> > > --
> > > 2.47.0.371.ga323438b13-goog
> > >
