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

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Message-ID: <3c471d06-d8c4-47a2-b29b-4faa5d61d25d@proton.me>
Date: Tue, 10 Sep 2024 07:10:46 +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 04.09.24 12:32, Alice Ryhl wrote:
> On Tue, Sep 3, 2024 at 12:17 AM Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>> 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.
> 
> It sounds like we'll just want a macro that generates a global wrapped
> in a Mutex/SpinLock for now ...

Yeah.

> If we're going to do that, we could take the extra step and have it
> generate special Guard and LockedBy types so that you can have a
> LockedBy that doesn't need to make any runtime checks.

Oh that is a good idea!

---
Cheers,
Benno