linux-kernel - Re: [PATCH v2] rust: add global lock support

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Message-ID: <0030a292-49f4-4575-846f-424b098c7f1a@proton.me>
Date: Mon, 02 Sep 2024 22:16:53 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Andreas Hindborg <a.hindborg@...sung.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rust: add global lock support

On 02.09.24 13:42, Alice Ryhl wrote:
> On Mon, Sep 2, 2024 at 1:37 PM Alice Ryhl <aliceryhl@...gle.com> wrote:
>>
>> On Fri, Aug 30, 2024 at 3:22 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>>
>>> On 30.08.24 07:34, Alice Ryhl wrote:
>>>> On Thu, Aug 29, 2024 at 8:17 PM Benno Lossin <benno.lossin@...ton.me> wrote:
>>>>>
>>>>> On 27.08.24 10:41, Alice Ryhl wrote:
>>>>>> For architectures that don't use all-zeros for the unlocked case, we
>>>>>> will most likely have to hard-code the correct representation on the
>>>>>> Rust side.
>>>>>
>>>>> You mean in `unsafe_const_init`?
>>>>
>>>> No, I mean we would have `unsafe_const_new` directly set `state` to
>>>> the right value and let `unsafe_const_init` be a no-op.
>>>
>>> But how do you set the right value of a list_head? The value will be
>>> moved.
>>
>> Right ... we probably can't get around needing a macro. Can statics
>> even reference themselves?
> 
> Looks like they can:
> 
> use std::ptr::addr_of;
> 
> struct MyStruct {
>     ptr: *const MyStruct,
> }
> 
> static mut MY_STRUCT: MyStruct = MyStruct {
>     ptr: addr_of!(MY_STRUCT),
> };

That's useful to know...
But I don't see a way to get pinned-init to work with this. I would need
a lot of currently experimental features (const closures, const traits)
and a way to initialize a static without providing a direct value, since
I can't just do

    static mut MY_STRUCT: MyStruct = {
        unsafe { __pinned_init(addr_of_mut!(MY_STRUCT), /* initializer */) };
        unsafe { addr_of!(MY_STRUCT).read() }
    };

It (rightfully) complains that I am initializing the static with itself.

We /might/ be able to do something special for `Mutex`/ other locks, but
I haven't tried yet. So the unsafe approach seems the best at the moment.

---
Cheers,
Benno