Christian Brauner <brauner@xxxxxxxxxx> writes: > On Thu, Nov 30, 2023 at 09:17:56AM +0000, Alice Ryhl wrote: >> Christian Brauner <brauner@xxxxxxxxxx> writes: >>>>>> + /// Prevent values of this type from being moved to a different task. >>>>>> + /// >>>>>> + /// This is necessary because the C FFI calls assume that `current` is set to the task that >>>>>> + /// owns the fd in question. >>>>>> + _not_send_sync: PhantomData<*mut ()>, >>>>> >>>>> I don't fully understand this. Can you explain in a little more detail >>>>> what you mean by this and how this works? >>>> >>>> Yeah, so, this has to do with the Rust trait `Send` that controls >>>> whether it's okay for a value to get moved from one thread to another. >>>> In this case, we don't want it to be `Send` so that it can't be moved to >>>> another thread, since current might be different there. >>>> >>>> The `Send` trait is automatically applied to structs whenever *all* >>>> fields of the struct are `Send`. So to ensure that a struct is not >>>> `Send`, you add a field that is not `Send`. >>>> >>>> The `PhantomData` type used here is a special zero-sized type. >>>> Basically, it says "pretend this struct has a field of type `*mut ()`, >>>> but don't actually add the field". So for the purposes of `Send`, it has >>>> a non-Send field, but since its wrapped in `PhantomData`, the field is >>>> not there at runtime. >>> >>> This probably a stupid suggestion, question. But while PhantomData gives >>> the right hint of what is happening I wouldn't mind if that was very >>> explicitly called NoSendTrait or just add the explanatory comment. Yes, >>> that's a lot of verbiage but you'd help us a lot. >> >> I suppose we could add a typedef: >> >> type NoSendTrait = PhantomData<*mut ()>; >> >> and use that as the field type. The way I did it here is the "standard" >> way of doing it, and if you look at code outside the kernel, you will >> also find them using `PhantomData` like this. However, I don't mind >> adding the typedef if you think it is helpful. > > I'm fine with just a comment as well. I just need to be able to read > this a bit faster. I'm basically losing half a day just dealing with > this patchset and that's not realistic if I want to keep up with other > patches that get sent. > > And if you resend and someone else review you might have to answer the > same question again. What do you think about this wording? /// Prevent values of this type from being moved to a different task. /// /// This field has the type `PhantomData<*mut ()>`, which does not /// implement the Send trait. By adding a field with this property, we /// ensure that the `FileDescriptorReservation` struct will not /// implement the Send trait either. This has the consequence that the /// compiler will prevent you from moving values of type /// `FileDescriptorReservation` into a different task, which we want /// because other tasks might have a different value of `current`. We /// want to avoid that because `fd_install` assumes that the value of /// `current` is unchanged since the call to `get_unused_fd_flags`. /// /// The `PhantomData` type has size zero, so the field does not exist at /// runtime. Alice
