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.
