Re: [PATCH v4] rust: add global lock support

[Alice Ryhl]

On Thu, Oct 10, 2024 at 12:39 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> On 30.09.24 15:11, Alice Ryhl wrote:
> > diff --git a/rust/kernel/sync/lock/global.rs b/rust/kernel/sync/lock/global.rs
> > new file mode 100644
> > index 000000000000..fc02fac864f6
> > --- /dev/null
> > +++ b/rust/kernel/sync/lock/global.rs
> > @@ -0,0 +1,260 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +// Copyright (C) 2024 Google LLC.
> > +
> > +//! Support for defining statics containing locks.
> > +
> > +/// Defines a global lock.
> > +///
> > +/// Supports the following options:
> > +///
> > +/// * `value` specifies the initial value in the global lock.
> > +/// * `wrapper` specifies the name of the wrapper struct.
> > +/// * `guard` specifies the name of the guard type.
> > +/// * `locked_by` specifies the name of the `LockedBy` type.
> > +///
> > +/// # Examples
> > +///
> > +/// A global counter.
> > +///
> > +/// ```
> > +/// # mod ex {
> > +/// # use kernel::prelude::*;
> > +/// kernel::sync::global_lock! {
> > +///     // SAFETY: Initialized in module initializer before first use.
> > +///     static MY_COUNTER: Mutex<u32> = unsafe { uninit };
> > +///     value: 0;
> > +/// }
> > +///
> > +/// fn increment_counter() -> u32 {
> > +///     let mut guard = MY_COUNTER.lock();
> > +///     *guard += 1;
> > +///     *guard
> > +/// }
> > +///
> > +/// impl kernel::Module for MyModule {
> > +///     fn init(_module: &'static ThisModule) -> Result<Self> {
> > +///         // SAFETY: called exactly once
> > +///         unsafe { MY_COUNTER.init() };
> > +///
> > +///         Ok(MyModule {})
> > +///     }
> > +/// }
> > +/// # struct MyModule {}
> > +/// # }
> > +/// ```
> > +///
> > +/// A global mutex used to protect all instances of a given struct.
> > +///
> > +/// ```
> > +/// # mod ex {
> > +/// # use kernel::prelude::*;
> > +/// kernel::sync::global_lock! {
> > +///     // SAFETY: Initialized in module initializer before first use.
> > +///     static MY_MUTEX: Mutex<()> = unsafe { uninit };
> > +///     value: ();
> > +///     guard: MyGuard;
> > +///     locked_by: LockedByMyMutex;
> > +/// }
> > +///
> > +/// /// All instances of this struct are protected by `MY_MUTEX`.
> > +/// struct MyStruct {
> > +///     my_counter: LockedByMyMutex<u32>,
> > +/// }
> > +///
> > +/// impl MyStruct {
> > +///     /// Increment the counter in this instance.
> > +///     ///
> > +///     /// The caller must hold the `MY_MUTEX` mutex.
> > +///     fn increment(&self, guard: &mut MyGuard) -> u32 {
> > +///         let my_counter = self.my_counter.as_mut(guard);
> > +///         *my_counter += 1;
> > +///         *my_counter
> > +///     }
> > +/// }
> > +///
> > +/// impl kernel::Module for MyModule {
> > +///     fn init(_module: &'static ThisModule) -> Result<Self> {
> > +///         // SAFETY: called exactly once
> > +///         unsafe { MY_MUTEX.init() };
> > +///
> > +///         Ok(MyModule {})
> > +///     }
> > +/// }
> > +/// # struct MyModule {}
> > +/// # }
> > +/// ```
>
> The docs here don't mention that you still need to call `.init()`
> manually (though the examples show it nicely). I don't know if we want
> macros to have a `# Safety` section.
>
> > +#[macro_export]
> > +macro_rules! global_lock {
> > +    {
> > +        $(#[$meta:meta])* $pub:vis static $name:ident: $kind:ident<$valuety:ty> = unsafe { uninit };
> > +        value: $value:expr;
>
> I would find it more natural to use `=` instead of `:` here, since then
> it would read as a normal statement with the semicolon at the end.
> Another alternative would be to use `,` instead of `;`, but that doesn't
> work nicely with the static keyword above (although you could make the
> user write it in another {}, but that also isn't ideal...).
>
> Using `=` instead of `:` makes my editor put the correct amount of
> indentation there, `:` adds a lot of extra spaces.

That seems sensible.

> > +        wrapper: $wrapper:ident;
> > +        $( name: $lname:literal; )?
> > +        $(
> > +            guard: $guard:ident;
> > +            locked_by: $locked_by:ident;
> > +        )?
> > +    } => {
> > +        $crate::macros::paste! {
> > +            type [< __static_lock_ty_ $name >] = $valuety;
> > +            const [< __static_lock_init_ $name >]: [< __static_lock_ty_ $name >] = $value;
>
> Why are these two items outside of the `mod` below?
> Also why do you need to define the type alias? You could just use
> `$valuety`, right?

Because they might access things that are in scope here, but not in
scope inside the module.

> Also,
>
>     error: type `__static_lock_ty_VALUE` should have an upper camel case name
>        --> rust/kernel/sync/lock/global.rs:100:18
>         |
>     100 |               type [< __static_lock_ty_ $name >] = $valuety;
>         |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: convert the identifier to upper camel case: `StaticLockTyValue`
>
> The same error affects the `wrapper` type forwarding below.
>
>
> > +
> > +            #[allow(unused_pub)]
>
>     error: unknown lint: `unused_pub`
>        --> rust/kernel/sync/lock/global.rs:103:21
>         |
>     103 |               #[allow(unused_pub)]
>         |                       ^^^^^^^^^^ help: did you mean: `unused_mut`

Uhhh. This is the lint for when you mark a function pub but don't
actually export it from the crate. But now I can't find the lint
anywhere ... I'm so confused.

> Though I also get
>
>     error: methods `init` and `lock` are never used
>        --> rust/kernel/sync/lock/global.rs:128:42
>         |
>     122 | /                 impl $wrapper {
>     123 | |                     /// Initialize the global lock.
>     124 | |                     ///
>     125 | |                     /// # Safety
>     ...   |
>     128 | |                     pub(crate) unsafe fn init(&'static self) {
>         | |                                          ^^^^
>     ...   |
>     142 | |                     pub(crate) fn lock(&'static self) -> $crate::global_lock_inner!(guard $kind, $valuety $(, $guard)?) {
>         | |                                   ^^^^
>     ...   |
>     146 | |                     }
>     147 | |                 }
>         | |_________________- methods in this implementation
>
> But that is governed by the `dead_code` lint.

I guess I have to look into the lints again. I did not get this lint.

> > +            mod [< __static_lock_mod_ $name >] {
> > +                use super::[< __static_lock_ty_ $name >] as Val;
> > +                use super::[< __static_lock_init_ $name >] as INIT;
> > +                type Backend = $crate::global_lock_inner!(backend $kind);
> > +                type GuardTyp = $crate::global_lock_inner!(guard $kind, Val $(, $guard)?);
>
> `GuardTyp` is only used once, so you should be able to just inline it.
> `Backend` is used twice, but I don't know if we need a type alias for
> it.

They're both used twice. Inlining them makes the lines really long.

> > +
> > +                /// # Safety
> > +                ///
> > +                /// Must be used to initialize `super::$name`.
> > +                pub(super) const unsafe fn new() -> $wrapper {
>
> Why is this function not associated to `$wrapper`?
>
> > +                    let state = $crate::types::Opaque::uninit();
>
> Why not add
>
>     const INIT: $valuety = $value;
>
> here instead of outside the mod.

Because it might reference things that are only in scope outside of the module.

> > +                    $wrapper {
> > +                        // SAFETY: The user of this macro promises to call `init` before calling
> > +                        // `lock`.
> > +                        inner: unsafe {
> > +                            $crate::sync::lock::Lock::global_lock_helper_new(state, INIT)
> > +                        }
> > +                    }
> > +                }
> > +
> > +                /// Wrapper type for a global lock.
> > +                pub(crate) struct $wrapper {
>
> How can the wrapper struct be `pub(crate)` when the constant might be
> global `pub`?
>
>     error: type `__static_lock_wrapper_INIT` is more private than the item `INIT`
>        --> rust/kernel/sync/lock/global.rs:206:14
>         |
>     206 |               };
>         |                ^ static `INIT` is reachable at visibility `pub`
>         |
>
> The functions should probably just be `pub`.

I used to do that, but got some errors about `pub` being unused. I'll
look into this again.

Alice