mango-hal 0.2.1

// SPDX-License-Identifier: MIT OR Apache-2.0
//
// Copyright (c) 2020-2023 Andre Richter <andre.o.richter@gmail.com>

//! Synchronization primitives.
//!
//! # Resources
//!
//!   - <https://doc.rust-lang.org/book/ch16-04-extensible-concurrency-sync-and-send.html>
//!   - <https://stackoverflow.com/questions/59428096/understanding-the-send-trait>
//!   - <https://doc.rust-lang.org/std/cell/index.html>

use core::cell::UnsafeCell;

//--------------------------------------------------------------------------------------------------
// Public Definitions
//--------------------------------------------------------------------------------------------------

/// Synchronization interfaces.
pub mod interface
{

  /// Any object implementing this trait guarantees exclusive access to the data wrapped within
  /// the Mutex for the duration of the provided closure.
  pub trait Mutex
  {
    /// The type of the data that is wrapped by this mutex.
    type Data;

    /// Locks the mutex and grants the closure temporary mutable access to the wrapped data.
    fn lock<'a, R>(&'a self, f: impl FnOnce(&'a mut Self::Data) -> R) -> R;
  }

  /// A reader-writer exclusion type.
  ///
  /// The implementing object allows either a number of readers or at most one writer at any point
  /// in time.
  pub trait ReadWriteEx
  {
    /// The type of encapsulated data.
    type Data;

    /// Grants temporary mutable access to the encapsulated data.
    fn write<'a, R>(&'a self, f: impl FnOnce(&'a mut Self::Data) -> R) -> R;

    /// Grants temporary immutable access to the encapsulated data.
    fn read<'a, R>(&'a self, f: impl FnOnce(&'a Self::Data) -> R) -> R;
  }
}

/// A pseudo-lock for teaching purposes.
///
/// In contrast to a real Mutex implementation, does not protect against concurrent access from
/// other cores to the contained data. This part is preserved for later lessons.
///
/// The lock will only be used as long as it is safe to do so, i.e. as long as the kernel is
/// executing on a single core.
pub struct IRQSafeNullLock<T>
where
  T: ?Sized,
{
  data: UnsafeCell<T>,
}

/// A pseudo-lock that is RW during the single-core kernel init phase and RO afterwards.
///
/// Intended to encapsulate data that is populated during kernel init when no concurrency exists.
pub struct InitStateLock<T>
where
  T: ?Sized,
{
  data: UnsafeCell<T>,
}

//--------------------------------------------------------------------------------------------------
// Public Code
//--------------------------------------------------------------------------------------------------

unsafe impl<T> Send for IRQSafeNullLock<T> where T: ?Sized + Send {}
unsafe impl<T> Sync for IRQSafeNullLock<T> where T: ?Sized + Send {}

impl<T> IRQSafeNullLock<T>
{
  /// Create an instance.
  pub const fn new(data: T) -> Self
  {
    Self {
      data: UnsafeCell::new(data),
    }
  }
}

unsafe impl<T> Send for InitStateLock<T> where T: ?Sized + Send {}
unsafe impl<T> Sync for InitStateLock<T> where T: ?Sized + Send {}

impl<T> InitStateLock<T>
{
  /// Create an instance.
  pub const fn new(data: T) -> Self
  {
    Self {
      data: UnsafeCell::new(data),
    }
  }
}

//------------------------------------------------------------------------------
// OS Interface Code
//------------------------------------------------------------------------------
use super::{exception, state};

impl<T> interface::Mutex for IRQSafeNullLock<T>
{
  type Data = T;

  fn lock<'a, R>(&'a self, f: impl FnOnce(&'a mut Self::Data) -> R) -> R
  {
    // In a real lock, there would be code encapsulating this line that ensures that this
    // mutable reference will ever only be given out once at a time.
    let data = unsafe { &mut *self.data.get() };

    // Execute the closure while IRQs are masked.
    exception::asynchronous::exec_with_irq_masked(|| f(data))
  }
}

impl<T> interface::ReadWriteEx for InitStateLock<T>
{
  type Data = T;

  fn write<'a, R>(&'a self, f: impl FnOnce(&'a mut Self::Data) -> R) -> R
  {
    assert!(
      state::state_manager().is_init(),
      "InitStateLock::write called after kernel init phase"
    );
    assert!(
      !exception::asynchronous::is_local_irq_masked(),
      "InitStateLock::write called with IRQs unmasked"
    );

    let data = unsafe { &mut *self.data.get() };

    f(data)
  }

  fn read<'a, R>(&'a self, f: impl FnOnce(&'a Self::Data) -> R) -> R
  {
    let data = unsafe { &*self.data.get() };

    f(data)
  }
}

//--------------------------------------------------------------------------------------------------
// Testing
//--------------------------------------------------------------------------------------------------

#[cfg(test)]
mod tests
{
  use super::*;

  /// InitStateLock must be transparent.
  #[test_case]
  fn init_state_lock_is_transparent()
  {
    use core::mem::size_of;

    assert_eq!(size_of::<InitStateLock<u64>>(), size_of::<u64>());
  }
}