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#![cfg_attr(not(feature = "std"), no_std)]
#![doc = include_str!("../README.md")]
mod mutex;
#[cfg(feature = "std")]
mod std;
use core::marker::PhantomData;
pub use self::mutex::Mutex;
/// Critical section token.
///
/// An instance of this type indicates that the current thread is executing code within a critical
/// section.
#[derive(Clone, Copy, Debug)]
pub struct CriticalSection<'cs> {
_private: PhantomData<&'cs ()>,
}
impl<'cs> CriticalSection<'cs> {
/// Creates a critical section token.
///
/// This method is meant to be used to create safe abstractions rather than being directly used
/// in applications.
///
/// # Safety
///
/// This must only be called when the current thread is in a critical section. The caller must
/// ensure that the returned instance will not live beyond the end of the critical section.
///
/// The caller must use adequate fences to prevent the compiler from moving the
/// instructions inside the critical section to the outside of it. Sequentially consistent fences are
/// suggested immediately after entry and immediately before exit from the critical section.
///
/// Note that the lifetime `'cs` of the returned instance is unconstrained. User code must not
/// be able to influence the lifetime picked for this type, since that might cause it to be
/// inferred to `'static`.
#[inline(always)]
pub unsafe fn new() -> Self {
CriticalSection {
_private: PhantomData,
}
}
}
#[cfg(any(
all(feature = "restore-state-none", feature = "restore-state-bool"),
all(feature = "restore-state-none", feature = "restore-state-u8"),
all(feature = "restore-state-none", feature = "restore-state-u16"),
all(feature = "restore-state-none", feature = "restore-state-u32"),
all(feature = "restore-state-none", feature = "restore-state-u64"),
all(feature = "restore-state-bool", feature = "restore-state-u8"),
all(feature = "restore-state-bool", feature = "restore-state-u16"),
all(feature = "restore-state-bool", feature = "restore-state-u32"),
all(feature = "restore-state-bool", feature = "restore-state-u64"),
all(feature = "restore-state-u8", feature = "restore-state-u16"),
all(feature = "restore-state-u8", feature = "restore-state-u32"),
all(feature = "restore-state-u8", feature = "restore-state-u64"),
all(feature = "restore-state-u16", feature = "restore-state-u32"),
all(feature = "restore-state-u16", feature = "restore-state-u64"),
all(feature = "restore-state-u32", feature = "restore-state-u64"),
))]
compile_error!("You must set at most one of these Cargo features: restore-state-none, restore-state-bool, restore-state-u8, restore-state-u16, restore-state-u32, restore-state-u64");
#[cfg(not(any(
feature = "restore-state-bool",
feature = "restore-state-u8",
feature = "restore-state-u16",
feature = "restore-state-u32",
feature = "restore-state-u64"
)))]
type RawRestoreStateInner = ();
#[cfg(feature = "restore-state-bool")]
type RawRestoreStateInner = bool;
#[cfg(feature = "restore-state-u8")]
type RawRestoreStateInner = u8;
#[cfg(feature = "restore-state-u16")]
type RawRestoreStateInner = u16;
#[cfg(feature = "restore-state-u32")]
type RawRestoreStateInner = u32;
#[cfg(feature = "restore-state-u64")]
type RawRestoreStateInner = u64;
// We have RawRestoreStateInner and RawRestoreState so that we don't have to copypaste the docs 5 times.
// In the docs this shows as `pub type RawRestoreState = u8` or whatever the selected type is, because
// the "inner" type alias is private.
/// Raw, transparent "restore state".
///
/// This type changes based on which Cargo feature is selected, out of
/// - `restore-state-none` (default, makes the type be `()`)
/// - `restore-state-bool`
/// - `restore-state-u8`
/// - `restore-state-u16`
/// - `restore-state-u32`
/// - `restore-state-u64`
///
/// See [`RestoreState`].
///
/// User code uses [`RestoreState`] opaquely, critical section implementations
/// use [`RawRestoreState`] so that they can use the inner value.
pub type RawRestoreState = RawRestoreStateInner;
/// Opaque "restore state".
///
/// Implementations use this to "carry over" information between acquiring and releasing
/// a critical section. For example, when nesting two critical sections of an
/// implementation that disables interrupts globally, acquiring the inner one won't disable
/// the interrupts since they're already disabled. The impl would use the restore state to "tell"
/// the corresponding release that it does *not* have to reenable interrupts yet, only the
/// outer release should do so.
///
/// User code uses [`RestoreState`] opaquely, critical section implementations
/// use [`RawRestoreState`] so that they can use the inner value.
#[derive(Clone, Copy, Debug)]
pub struct RestoreState(RawRestoreState);
impl RestoreState {
/// Create an invalid, dummy `RestoreState`.
///
/// This can be useful to avoid `Option` when storing a `RestoreState` in a
/// struct field, or a `static`.
///
/// Note that due to the safety contract of [`acquire`]/[`release`], you must not pass
/// a `RestoreState` obtained from this method to [`release`].
pub const fn invalid() -> Self {
#[cfg(not(any(
feature = "restore-state-bool",
feature = "restore-state-u8",
feature = "restore-state-u16",
feature = "restore-state-u32",
feature = "restore-state-u64"
)))]
return Self(());
#[cfg(feature = "restore-state-bool")]
return Self(false);
#[cfg(feature = "restore-state-u8")]
return Self(0);
#[cfg(feature = "restore-state-u16")]
return Self(0);
#[cfg(feature = "restore-state-u32")]
return Self(0);
#[cfg(feature = "restore-state-u64")]
return Self(0);
}
}
/// Acquire a critical section in the current thread.
///
/// This function is extremely low level. Strongly prefer using [`with`] instead.
///
/// Nesting critical sections is allowed. The inner critical sections
/// are mostly no-ops since they're already protected by the outer one.
///
/// # Safety
///
/// - Each `acquire` call must be paired with exactly one `release` call in the same thread.
/// - `acquire` returns a "restore state" that you must pass to the corresponding `release` call.
/// - `acquire`/`release` pairs must be "properly nested", ie it's not OK to do `a=acquire(); b=acquire(); release(a); release(b);`.
/// - It is UB to call `release` if the critical section is not acquired in the current thread.
/// - It is UB to call `release` with a "restore state" that does not come from the corresponding `acquire` call.
/// - It must provide ordering guarantees at least equivalent to a [`core::sync::atomic::Ordering::Acquire`]
/// on a memory location shared by all critical sections, on which the `release` call will do a
/// [`core::sync::atomic::Ordering::Release`] operation.
#[inline(always)]
pub unsafe fn acquire() -> RestoreState {
extern "Rust" {
fn _critical_section_1_0_acquire() -> RawRestoreState;
}
#[allow(clippy::unit_arg)]
RestoreState(_critical_section_1_0_acquire())
}
/// Release the critical section.
///
/// This function is extremely low level. Strongly prefer using [`with`] instead.
///
/// # Safety
///
/// See [`acquire`] for the safety contract description.
#[inline(always)]
pub unsafe fn release(restore_state: RestoreState) {
extern "Rust" {
fn _critical_section_1_0_release(restore_state: RawRestoreState);
}
#[allow(clippy::unit_arg)]
_critical_section_1_0_release(restore_state.0)
}
/// Execute closure `f` in a critical section.
///
/// Nesting critical sections is allowed. The inner critical sections
/// are mostly no-ops since they're already protected by the outer one.
///
/// # Panics
///
/// This function panics if the given closure `f` panics. In this case
/// the critical section is released before unwinding.
#[inline]
pub fn with<R>(f: impl FnOnce(CriticalSection) -> R) -> R {
// Helper for making sure `release` is called even if `f` panics.
struct Guard {
state: RestoreState,
}
impl Drop for Guard {
#[inline(always)]
fn drop(&mut self) {
unsafe { release(self.state) }
}
}
let state = unsafe { acquire() };
let _guard = Guard { state };
unsafe { f(CriticalSection::new()) }
}
/// Methods required for a critical section implementation.
///
/// This trait is not intended to be used except when implementing a critical section.
///
/// # Safety
///
/// Implementations must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
pub unsafe trait Impl {
/// Acquire the critical section.
///
/// # Safety
///
/// Callers must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
unsafe fn acquire() -> RawRestoreState;
/// Release the critical section.
///
/// # Safety
///
/// Callers must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
unsafe fn release(restore_state: RawRestoreState);
}
/// Set the critical section implementation.
///
/// # Example
///
/// ```
/// # #[cfg(not(feature = "std"))] // needed for `cargo test --features std`
/// # mod no_std {
/// use critical_section::RawRestoreState;
///
/// struct MyCriticalSection;
/// critical_section::set_impl!(MyCriticalSection);
///
/// unsafe impl critical_section::Impl for MyCriticalSection {
/// unsafe fn acquire() -> RawRestoreState {
/// // ...
/// }
///
/// unsafe fn release(restore_state: RawRestoreState) {
/// // ...
/// }
/// }
/// # }
#[macro_export]
macro_rules! set_impl {
($t: ty) => {
#[no_mangle]
unsafe fn _critical_section_1_0_acquire() -> $crate::RawRestoreState {
<$t as $crate::Impl>::acquire()
}
#[no_mangle]
unsafe fn _critical_section_1_0_release(restore_state: $crate::RawRestoreState) {
<$t as $crate::Impl>::release(restore_state)
}
};
}