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

Andreas Hindborg
2024-10-10T13:57:52Z

Hi Alice,

Alice Ryhl <aliceryhl@google.com> writes:

> Add support for creating global variables that are wrapped in a mutex or
> spinlock. Optionally, the macro can generate a special LockedBy type
> that does not require a runtime check.
>
> The implementation here is intended to replace the global mutex
> workaround found in the Rust Binder RFC [1]. In both cases, the global
> lock must be initialized before first use. The macro is unsafe to use
> for the same reason.
>
> The separate initialization step is required because it is tricky to
> access the value of __ARCH_SPIN_LOCK_UNLOCKED from Rust. Doing so will
> require changes to the C side. That change will happen as a follow-up to
> this patch.

Why is this a challenge? It seems to work with locks that are not
global.

[...]

> 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.

Could you add an example to demonstrate when using `wrapper` option
would be useful?

> +/// * `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 {}
> +/// # }
> +/// ```
> +#[macro_export]
> +macro_rules! global_lock {
> + {
> + $(#[$meta:meta])* $pub:vis static $name:ident: $kind:ident<$valuety:ty> = unsafe { uninit };
> + value: $value:expr;
> + 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;
> +
> + #[allow(unused_pub)]
> + 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 $(, $guard)?);
> +
> + /// # Safety
> + ///
> + /// Must be used to initialize `super::$name`.
> + pub(super) const unsafe fn new() -> $wrapper {
> + let state = $crate::types::Opaque::uninit();
> + $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 {
> + inner: $crate::sync::lock::Lock<Val, Backend>,
> + }
> +
> + impl $wrapper {
> + /// Initialize the global lock.
> + ///
> + /// # Safety
> + ///
> + /// This method must not be called more than once.
> + pub(crate) unsafe fn init(&'static self) {
> + // SAFETY:
> + // * This type can only be created by `new`.
> + // * Caller promises to not call this method more than once.
> + unsafe {
> + $crate::sync::lock::Lock::global_lock_helper_init(
> + ::core::pin::Pin::static_ref(&self.inner),
> + $crate::optional_name!($($lname)?),
> + $crate::static_lock_class!(),
> + );
> + }
> + }
> +
> + /// Lock this global lock.
> + pub(crate) fn lock(&'static self) -> GuardTyp {
> + $crate::global_lock_inner!(new_guard $($guard)? {
> + self.inner.lock()
> + })
> + }
> +
> + /// Lock this global lock.

"Try to lock..." ?

Best regards,
Andreas