From: Benno Lossin <benno.lossin@proton.me>
To: Alice Ryhl <aliceryhl@google.com>
Cc: brauner@kernel.org, a.hindborg@samsung.com,
alex.gaynor@gmail.com, arve@android.com,
bjorn3_gh@protonmail.com, boqun.feng@gmail.com,
cmllamas@google.com, dan.j.williams@intel.com, dxu@dxuuu.xyz,
gary@garyguo.net, gregkh@linuxfoundation.org,
joel@joelfernandes.org, keescook@chromium.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
maco@android.com, ojeda@kernel.org, peterz@infradead.org,
rust-for-linux@vger.kernel.org, surenb@google.com,
tglx@linutronix.de, tkjos@android.com, viro@zeniv.linux.org.uk,
wedsonaf@gmail.com, willy@infradead.org
Subject: Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`
Date: Thu, 30 Nov 2023 12:17:14 +0000 [thread overview]
Message-ID: <nFDPJFnzE9Q5cqY7FwSMByRH2OAn_BpI4H53NQfWIlN6I2qfmAqnkp2wRqn0XjMO65OyZY4h6P4K2nAGKJpAOSzksYXaiAK_FoH_8QbgBI4=@proton.me> (raw)
In-Reply-To: <20231130115451.138496-1-aliceryhl@google.com>
On 30.11.23 12:54, Alice Ryhl wrote:
> Christian Brauner <brauner@kernel.org> writes:
>> On Thu, Nov 30, 2023 at 09:17:56AM +0000, Alice Ryhl wrote:
>>> Christian Brauner <brauner@kernel.org> 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,
--
Cheers,
Benno
next prev parent reply other threads:[~2023-11-30 12:17 UTC|newest]
Thread overview: 96+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-29 12:51 [PATCH 0/7] File abstractions needed by Rust Binder Alice Ryhl
2023-11-29 12:51 ` [PATCH 1/7] rust: file: add Rust abstraction for `struct file` Alice Ryhl
2023-11-29 15:13 ` Matthew Wilcox
2023-11-29 15:23 ` Peter Zijlstra
2023-11-29 17:08 ` Boqun Feng
2023-11-30 10:42 ` Peter Zijlstra
2023-11-30 15:25 ` Boqun Feng
2023-12-01 8:53 ` Peter Zijlstra
2023-12-01 9:19 ` Boqun Feng
2023-12-01 9:40 ` Peter Zijlstra
2023-12-01 10:36 ` Boqun Feng
2023-12-01 11:05 ` Peter Zijlstra
2023-12-01 9:00 ` Peter Zijlstra
2023-12-01 9:52 ` Boqun Feng
2023-11-29 16:42 ` Alice Ryhl
2023-11-29 16:45 ` Peter Zijlstra
2023-11-30 15:02 ` Benno Lossin
2023-11-29 17:06 ` Christian Brauner
2023-11-29 21:27 ` Alice Ryhl
2023-11-29 23:17 ` Benno Lossin
2023-11-30 10:48 ` Christian Brauner
2023-11-30 12:10 ` Alice Ryhl
2023-11-30 12:36 ` Christian Brauner
2023-11-30 14:53 ` Benno Lossin
2023-11-30 14:59 ` Greg Kroah-Hartman
2023-11-30 15:46 ` Benno Lossin
2023-11-30 15:56 ` Greg Kroah-Hartman
2023-11-30 15:58 ` Theodore Ts'o
2023-11-30 16:12 ` Benno Lossin
2023-12-01 1:16 ` Theodore Ts'o
2023-12-01 12:11 ` David Laight
2023-12-01 12:27 ` Alice Ryhl
2023-12-01 15:04 ` Theodore Ts'o
2023-12-01 15:14 ` Benno Lossin
2023-12-01 17:25 ` David Laight
2023-12-01 17:37 ` Benno Lossin
2023-11-29 12:51 ` [PATCH 2/7] rust: cred: add Rust abstraction for `struct cred` Alice Ryhl
2023-11-30 16:17 ` Benno Lossin
2023-12-01 9:06 ` Alice Ryhl
2023-12-01 10:27 ` Christian Brauner
2023-12-04 15:42 ` Alice Ryhl
2023-11-29 13:11 ` [PATCH 3/7] rust: security: add abstraction for secctx Alice Ryhl
2023-11-30 16:26 ` Benno Lossin
2023-12-01 10:48 ` Alice Ryhl
2023-12-02 10:03 ` Benno Lossin
2023-11-29 13:11 ` [PATCH 4/7] rust: file: add `FileDescriptorReservation` Alice Ryhl
2023-11-29 16:14 ` Christian Brauner
2023-11-29 16:55 ` Alice Ryhl
2023-11-29 17:14 ` Alice Ryhl
2023-11-30 9:12 ` Christian Brauner
2023-11-30 9:23 ` Alice Ryhl
2023-11-30 9:09 ` Christian Brauner
2023-11-30 9:17 ` Alice Ryhl
2023-11-30 10:51 ` Christian Brauner
2023-11-30 11:54 ` Alice Ryhl
2023-11-30 12:17 ` Benno Lossin [this message]
2023-11-30 12:33 ` Christian Brauner
2023-11-30 16:40 ` Benno Lossin
2023-12-01 11:32 ` Alice Ryhl
2023-11-29 13:12 ` [PATCH 5/7] rust: file: add `Kuid` wrapper Alice Ryhl
2023-11-29 16:28 ` Christian Brauner
2023-11-29 16:48 ` Peter Zijlstra
2023-11-30 12:46 ` Christian Brauner
2023-12-06 19:59 ` Kent Overstreet
2023-12-08 16:26 ` Peter Zijlstra
2023-12-08 19:58 ` Kent Overstreet
2023-12-08 5:28 ` comex
2023-12-08 16:19 ` Miguel Ojeda
2023-12-08 17:08 ` Nick Desaulniers
2023-12-08 17:37 ` Miguel Ojeda
2023-12-08 17:43 ` Boqun Feng
2023-12-08 20:43 ` Matthew Wilcox
2023-12-09 7:24 ` comex
2023-11-30 9:36 ` Alice Ryhl
2023-11-30 10:52 ` Christian Brauner
2023-11-30 10:36 ` Peter Zijlstra
2023-12-06 20:02 ` Kent Overstreet
2023-12-07 7:18 ` Greg Kroah-Hartman
2023-12-07 7:46 ` Kent Overstreet
2023-11-30 16:48 ` Benno Lossin
2023-11-29 13:12 ` [PATCH 6/7] rust: file: add `DeferredFdCloser` Alice Ryhl
2023-11-30 17:12 ` Benno Lossin
2023-12-01 11:35 ` Alice Ryhl
2023-12-02 10:16 ` Benno Lossin
2023-12-05 14:43 ` Alice Ryhl
2023-12-05 18:16 ` Alice Ryhl
2023-11-29 13:12 ` [PATCH 7/7] rust: file: add abstraction for `poll_table` Alice Ryhl
2023-11-30 17:42 ` Benno Lossin
2023-12-01 11:47 ` Alice Ryhl
2023-11-30 22:39 ` Boqun Feng
2023-12-01 11:50 ` Alice Ryhl
2023-11-30 22:50 ` Boqun Feng
2023-11-29 16:31 ` [PATCH 0/7] File abstractions needed by Rust Binder Christian Brauner
2023-11-29 16:48 ` Miguel Ojeda
2023-12-06 20:05 ` Kent Overstreet
2023-12-08 16:59 ` Miguel Ojeda
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