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

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Message-ID: <CAH5fLgh9kHm4XbcH2X4y6nwZ9VLYuUeu3hNmQBcdZ+Vx1H1L9w@mail.gmail.com>
Date: Wed, 4 Sep 2024 12:32:47 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Benno Lossin <benno.lossin@...ton.me>
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 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 ...

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.

Alice