On Thu, Nov 30, 2023 at 12:17:14PM +0000, Benno Lossin wrote: > On 30.11.23 12:54, Alice Ryhl wrote: > > 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 > > I don't think it is a good idea to add this big comment to every > `PhantomData` field. I would much rather have a type alias: > > /// Zero-sized type to mark types not [`Send`]. > /// > /// Add this type as a field to your struct if your type should not be sent to a different task. > /// Since [`Send`] is an auto trait, adding a single field that is [`!Send`] will ensure that the > /// whole type is [`!Send`]. > /// > /// If a type is [`!Send`] it is impossible to give control over an instance of the type to another > /// task. This is useful when a type stores task-local information for example file descriptors. > pub type NotSend = PhantomData<*mut ()>; > > If you have suggestions for improving the doc comment or the name, > please go ahead. > > This doesn't mean that there should be no comment on the `NotSend` > field of `FileDescriptorReservation`, but I don't want to repeat > the `Send` stuff all over the place (since it comes up a lot): > > /// Ensure that `FileDescriptorReservation` cannot be sent to a different task, since there the > /// value of `current` is different. We want to avoid that because `fd_install` assumes that the > /// value of `current` is unchanged since the call to `get_unused_fd_flags`. > _not_send: NotSend, Seems sane to me. But I would suggest to move away from the "send" terminology? * CurrentOnly * AccessCurrentTask vs AccessForeignTask * NoForeignTaskAccess * TaskLocalContext * TaskCurrentAccess Or some other variant thereof.
