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//! A rendezvous-based channel for synchronous Inter-Task Communication (ITC).
//!
//! This crate offers a rendezvous channel, in which two tasks can exchange messages
//! without an intermediary buffer.
//! The sender and receiver tasks must rendezvous together to exchange data,
//! so at least one of them must block.
//!
//! Only `Send` types can be sent or received through the channel.
//!
//! This is not a zero-copy channel; to avoid copying large messages,
//! use a reference type like `Box` or another layer of indirection.
//!
//! TODO: add support for a queue of pending senders and receivers
//! so that we can enable MPMC (multi-producer multi-consumer) behavior
//! that allows senders and receivers to be cloned.
//! Note that currently only a single receiver and single sender is supported.
#![no_std]
extern crate alloc;
#[macro_use] extern crate log;
#[cfg(trace_channel)]
#[macro_use] extern crate debugit;
extern crate spin;
extern crate sync_irq;
extern crate wait_queue;
extern crate wait_guard;
extern crate task;
extern crate scheduler;
extern crate sync;
extern crate sync_spin;
use core::fmt;
use alloc::sync::Arc;
use sync_irq::IrqSafeMutex;
use spin::Mutex;
use wait_queue::WaitQueue;
use wait_guard::WaitGuard;
use sync::DeadlockPrevention;
use sync_spin::Spin;
/// A wrapper type for an `ExchangeSlot` that is used for sending only.
struct SenderSlot<T>(Arc<IrqSafeMutex<ExchangeState<T>>>);
/// A wrapper type for an `ExchangeSlot` that is used for receiving only.
struct ReceiverSlot<T>(Arc<IrqSafeMutex<ExchangeState<T>>>);
/// An `ExchangeSlot` consists of two references to a shared state
/// that is used to exchange a message.
///
/// There is a "sender" reference and a "receiver" reference,
/// which are wrapped in their respective types: `SenderSlot` and `ReceiverSlot`.
struct ExchangeSlot<T> {
sender: Mutex<Option<SenderSlot<T>>>,
receiver: Mutex<Option<ReceiverSlot<T>>>,
}
impl<T> ExchangeSlot<T> {
fn new() -> ExchangeSlot<T> {
let inner = Arc::new(IrqSafeMutex::new(ExchangeState::Init));
ExchangeSlot {
sender: Mutex::new(Some(SenderSlot(inner.clone()))),
receiver: Mutex::new(Some(ReceiverSlot(inner))),
}
}
fn take_sender_slot(&self) -> Option<SenderSlot<T>> {
self.sender.lock().take()
}
fn take_receiver_slot(&self) -> Option<ReceiverSlot<T>> {
self.receiver.lock().take()
}
fn replace_sender_slot(&self, s: SenderSlot<T>) {
let _old = self.sender.lock().replace(s);
if _old.is_some() {
error!("BUG: REPLACE SENDER SLOT WAS SOME ALREADY");
}
}
fn replace_receiver_slot(&self, r: ReceiverSlot<T>) {
let _old = self.receiver.lock().replace(r);
if _old.is_some() {
error!("BUG: REPLACE RECEIVER SLOT WAS SOME ALREADY");
}
}
}
/// The possible states of an exchange slot in a rendezvous channel.
/// TODO: we should improve this state machine using session types
/// to check for valid state transitions at compile time.
enum ExchangeState<T> {
/// Initial state: we're waiting for either a sender or a receiver.
Init,
/// A sender has arrived before a receiver.
/// The `WaitGuard` contains the blocked sender task,
/// and the `T` is the message that will be exchanged.
WaitingForReceiver(WaitGuard, T),
/// A receiver has arrived before a sender.
/// The `WaitGuard` contains the blocked receiver task.
WaitingForSender(WaitGuard),
/// Sender and Receiver have rendezvoused, and the receiver finished first.
/// Thus, it is the sender's responsibility to reset to the initial state.
ReceiverFinishedFirst,
/// Sender and Receiver have rendezvoused, and the sender finished first.
/// Thus, the message `T` is enclosed here for the receiver to take,
/// and it is the receivers's responsibility to reset to the initial state.
SenderFinishedFirst(T),
}
impl<T> fmt::Debug for ExchangeState<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "ExchangeState::{}", match self {
ExchangeState::Init => "Init",
ExchangeState::WaitingForReceiver(..) => "WaitingForReceiver",
ExchangeState::WaitingForSender(..) => "WaitingForSender",
ExchangeState::ReceiverFinishedFirst => "ReceiverFinishedFirst",
ExchangeState::SenderFinishedFirst(..) => "SenderFinishedFirst",
})
}
}
// enum RendezvousState<T> {
// /// Initial state: we're waiting for either a sender or a receiver.
// Init,
// /// A task is blocked and waiting to rendezvous; the blocked task is held in the `WaitGuard`.
// /// The `Option<T>` is for exchanging the message, and indicates whether the blocked task is a sender or receiver.
// /// * If `None`, then the receiver is blocked, waiting on a sender to put its message into `Some(T)`.
// /// * If `Some`, then the sender is blocked, waiting on a receiver to take the message out of `Option<T>`.
// Waiting(WaitGuard, Option<T>),
// }
/// Creates a new rendezvous channel with the default deadlock prevention method.
///
/// For the vast majority of use cases, this function is recommended way to create
/// a new channel, because there is no need to specify a deadlock prevention method.
/// To create a channel with different deadlock prevention, see [`new_channel_with()`].
pub fn new_channel<T: Send>() -> (Sender<T>, Receiver<T>) {
new_channel_with()
}
/// Creates a new rendezvous channel with the specified deadlock prevention method.
///
/// See [`new_channel()`] for more details.
///
/// The rendezvous channel uses a wait queue internally and hence exposes a
/// deadlock prevention type parameter `P` that is [`Spin`] by default.
/// See [`WaitQueue`]'s documentation for more info on setting this type parameter.
pub fn new_channel_with<T: Send, P: DeadlockPrevention>() -> (Sender<T, P>, Receiver<T, P>) {
let channel = Arc::new(Channel {
slot: ExchangeSlot::new(),
waiting_senders: WaitQueue::new(),
waiting_receivers: WaitQueue::new(),
});
(
Sender { channel: channel.clone() },
Receiver { channel }
)
}
/// The inner channel for synchronous rendezvous-based communication
/// between `Sender`s and `Receiver`s.
///
/// This struct contains one or more exchange slot(s) (`ExchangeSlot`) as well as
/// queues for tasks that waiting to send or receive messages via those exchange slots.
///
/// Sender-side and Receiver-side references to an exchange slot can be obtained in both
/// a blocking and non-blocking fashion,
/// which supports both synchronous (rendezvous-based) and asynchronous channels.
struct Channel<T: Send, P: DeadlockPrevention = Spin> {
/// In a zero-capacity synchronous channel, there is only a single slot,
/// but senders and receivers perform a blocking wait on it until the slot becomes available.
/// In contrast, a synchronous channel with a capacity of 1 would return a "channel full" error
/// if the slot was taken, instead of blocking.
slot: ExchangeSlot<T>,
waiting_senders: WaitQueue<P>,
waiting_receivers: WaitQueue<P>,
}
impl<T: Send, P: DeadlockPrevention> Channel<T, P> {
/// Obtain a sender slot, blocking until one is available.
fn take_sender_slot(&self) -> SenderSlot<T> {
// Fast path: the uncontended case.
if let Some(s) = self.try_take_sender_slot() {
return s;
}
// Slow path: add ourselves to the waitqueue
// trace!("waiting to acquire sender slot...");
self.waiting_senders.wait_until(&|| self.try_take_sender_slot())
}
/// Obtain a receiver slot, blocking until one is available.
fn take_receiver_slot(&self) -> ReceiverSlot<T> {
// Fast path: the uncontended case.
if let Some(s) = self.try_take_receiver_slot() {
return s;
}
// Slow path: add ourselves to the waitqueue
// trace!("waiting to acquire receiver slot...");
self.waiting_receivers.wait_until(&|| self.try_take_receiver_slot())
}
/// Try to obtain a sender slot in a non-blocking fashion,
/// returning `None` if a slot is not immediately available.
fn try_take_sender_slot(&self) -> Option<SenderSlot<T>> {
self.slot.take_sender_slot()
}
/// Try to obtain a receiver slot in a non-blocking fashion,
/// returning `None` if a slot is not immediately available.
fn try_take_receiver_slot(&self) -> Option<ReceiverSlot<T>> {
self.slot.take_receiver_slot()
}
}
/// The sender (transmit) side of a channel.
#[derive(Clone)]
pub struct Sender<T: Send, P: DeadlockPrevention = Spin> {
channel: Arc<Channel<T, P>>,
}
impl <T: Send, P: DeadlockPrevention> Sender<T, P> {
/// Send a message, blocking until a receiver is ready.
///
/// Returns `Ok(())` if the message was sent and received successfully,
/// otherwise returns an error.
pub fn send(&self, msg: T) -> Result<(), &'static str> {
#[cfg(trace_channel)]
trace!("rendezvous: sending msg: {:?}", debugit!(msg));
// obtain a sender-side exchange slot, blocking if necessary
let sender_slot = self.channel.take_sender_slot();
// Here, either the sender (this task) arrived first and needs to wait for a receiver,
// or a receiver has already arrived and is waiting for a sender.
let retval = {
let mut exchange_state = sender_slot.0.lock();
// Temporarily take ownership of the channel's waiting state so we can modify it;
// the match statement below will advance the waiting state to the proper next state.
let current_state = core::mem::replace(&mut *exchange_state, ExchangeState::Init);
match current_state {
ExchangeState::Init => {
// Hold interrupts to avoid blocking & descheduling this task until we release the slot lock,
// which is currently done automatically because the slot uses a IrqSafeMutex.
let curr = task::get_my_current_task().ok_or("couldn't get current task")?;
*exchange_state = ExchangeState::WaitingForReceiver(
WaitGuard::new(curr).map_err(|_| "failed to create wait guard")?,
msg,
);
None
}
ExchangeState::WaitingForSender(receiver_to_notify) => {
// The message has been sent successfully.
*exchange_state = ExchangeState::SenderFinishedFirst(msg);
// Notify the receiver task (outside of this match statement),
// but DO NOT restore the sender slot to the channel yet;
// that will be done once the receiver is also finished with the slot (in SenderFinishedFirst).
Some(Ok(receiver_to_notify))
}
state => {
error!("BUG: Sender (at beginning) in invalid state {:?}", state);
*exchange_state = state;
Some(Err("BUG: Sender (at beginning) in invalid state"))
}
}
// here, the sender slot lock is dropped
};
// In the above block, we handled advancing the state of the exchange slot.
// Now we need to handle other stuff (like notifying waiters) without holding the sender_slot lock.
match retval {
Some(Ok(receiver_to_notify)) => {
drop(receiver_to_notify);
return Ok(());
}
Some(Err(e)) => {
// Restore the sender slot and notify waiting senders.
self.channel.slot.replace_sender_slot(sender_slot);
self.channel.waiting_senders.notify_one();
return Err(e);
}
None => {
scheduler::schedule();
}
}
// Here, the sender (this task) is waiting for a receiver
loop {
{
let exchange_state = sender_slot.0.lock();
match &*exchange_state {
ExchangeState::WaitingForReceiver(blocked_sender, ..) => {
if task::with_current_task(|t| t != blocked_sender.task())
.unwrap_or(true)
{
return Err("BUG: CURR TASK WAS DIFFERENT THAN BLOCKED SENDER");
}
blocked_sender.block_again().map_err(|_| "failed to block sender")?;
}
_ => break,
}
}
scheduler::schedule();
}
// Here, we are at the rendezvous point
let retval = {
let mut exchange_state = sender_slot.0.lock();
// Temporarily take ownership of the channel's waiting state so we can modify it;
// the match statement below will advance the waiting state to the proper next state.
let current_state = core::mem::replace(&mut *exchange_state, ExchangeState::Init);
match current_state {
ExchangeState::ReceiverFinishedFirst => {
// Ready to transfer another message.
*exchange_state = ExchangeState::Init;
Ok(())
}
state => {
error!("BUG: Sender (while waiting) in invalid state {:?}", state);
*exchange_state = state;
Err("BUG: Sender (while waiting) in invalid state")
}
}
};
if retval.is_ok() {
// Restore the receiver slot now that the receiver is finished, and notify waiting receivers.
self.channel.slot.replace_receiver_slot(ReceiverSlot(sender_slot.0.clone()));
self.channel.waiting_receivers.notify_one();
}
// Restore the sender slot and notify waiting senders.
// trace!("sender done, restoring slot");
self.channel.slot.replace_sender_slot(sender_slot);
self.channel.waiting_senders.notify_one();
// trace!("sender done, returning from send().");
retval
/*
loop {
let mut wait_entry = self.channel.waiter.lock();
// temporarily take ownership of the channel's waiting state so we can modify it.
let current_state = core::mem::replace(&mut *wait_entry, RendezvousState::Init);
match current_state {
RendezvousState::Init => {
let _held_interrupts = sync_irq::hold_interrupts();
*wait_entry = RendezvousState::Waiting(WaitGuard::new(curr_task.clone()), Some(msg));
// interrupts are re-enabled here
}
RendezvousState::Waiting(task_to_notify, dest) => {
*dest = Some(msg);
let _held_interrupts = sync_irq::hold_interrupts();
*task_to_notify = WaitGuard::new(curr_task.clone());
drop(task_to_notify); // notifies the receiver
}
};
let old_state = core::mem::replace(&mut wait_entry, new_state);
}
*/
}
/// Tries to send the message, only succeeding if a receiver is ready and waiting.
///
/// If a receiver was not ready, it returns the `msg` back to the caller without blocking.
///
/// Note that if the non-blocking `try_send` and `try_receive` functions are only ever used,
/// then the message will never be delivered because the sender and receiver cannot possibly rendezvous.
pub fn try_send(&self, _msg: T) -> Result<(), T> {
unimplemented!()
}
}
/// The receiver side of a channel.
#[derive(Clone)]
pub struct Receiver<T: Send, P: DeadlockPrevention = Spin> {
channel: Arc<Channel<T, P>>,
}
impl <T: Send, P: DeadlockPrevention> Receiver<T, P> {
/// Receive a message, blocking until a sender is ready.
///
/// Returns the message if it was received properly,
/// otherwise returns an error.
pub fn receive(&self) -> Result<T, &'static str> {
// trace!("rendezvous: receive() entry");
let curr_task = task::get_my_current_task().ok_or("couldn't get current task")?;
// obtain a receiver-side exchange slot, blocking if necessary
let receiver_slot = self.channel.take_receiver_slot();
// Here, either the receiver (this task) arrived first and needs to wait for a sender,
// or a sender has already arrived and is waiting for a receiver.
let retval = {
let mut exchange_state = receiver_slot.0.lock();
// Temporarily take ownership of the channel's waiting state so we can modify it;
// the match statement below will advance the waiting state to the proper next state.
let current_state = core::mem::replace(&mut *exchange_state, ExchangeState::Init);
match current_state {
ExchangeState::Init => {
// Hold interrupts to avoid blocking & descheduling this task until we release the slot lock,
// which is currently done automatically because the slot uses a IrqSafeMutex.
*exchange_state = ExchangeState::WaitingForSender(WaitGuard::new(curr_task.clone()).map_err(|_| "failed to create wait guard")?);
None
}
ExchangeState::WaitingForReceiver(sender_to_notify, msg) => {
// The message has been received successfully!
*exchange_state = ExchangeState::ReceiverFinishedFirst;
// Notify the sender task (outside of this match statement),
// but DO NOT restore the receiver slot to the channel yet;
// that will be done once the sender is also finished with the slot (in ReceiverFinishedFirst).
Some(Ok((sender_to_notify, msg)))
}
state => {
error!("BUG: Receiver (at beginning) in invalid state {:?}", state);
*exchange_state = state;
Some(Err("BUG: Receiver (at beginning) in invalid state"))
}
}
// here, the receiver slot lock is dropped
};
// In the above block, we handled advancing the state of the exchange slot.
// Now we need to handle other stuff (like notifying waiters) without holding the receiver_slot lock.
match retval {
Some(Ok((sender_to_notify, msg))) => {
drop(sender_to_notify);
#[cfg(trace_channel)]
trace!("rendezvous: received msg: {:?}", debugit!(msg));
return Ok(msg);
}
Some(Err(e)) => {
// Restore the receiver slot and notify waiting receivers.
self.channel.slot.replace_receiver_slot(receiver_slot);
self.channel.waiting_receivers.notify_one();
return Err(e);
}
None => {
scheduler::schedule();
}
}
// Here, the receiver (this task) is waiting for a sender
loop {
{
let exchange_state = receiver_slot.0.lock();
match &*exchange_state {
ExchangeState::WaitingForSender(blocked_receiver) => {
warn!("spurious wakeup while receiver is WaitingForSender... re-blocking task.");
if blocked_receiver.task() != &curr_task {
return Err("BUG: CURR TASK WAS DIFFERENT THAN BLOCKED RECEIVER");
}
blocked_receiver.block_again().map_err(|_| "failed to block receiver")?;
}
_ => break,
}
}
scheduler::schedule();
}
// Here, we are at the rendezvous point
let retval = {
let mut exchange_state = receiver_slot.0.lock();
// Temporarily take ownership of the channel's waiting state so we can modify it;
// the match statement below will advance the waiting state to the proper next state.
let current_state = core::mem::replace(&mut *exchange_state, ExchangeState::Init);
match current_state {
ExchangeState::SenderFinishedFirst(msg) => {
// Ready to transfer another message.
*exchange_state = ExchangeState::Init;
Ok(msg)
}
state => {
error!("BUG: Receiver (at end) in invalid state {:?}", state);
*exchange_state = state;
Err("BUG: Receiver (at end) in invalid state")
}
}
};
if retval.is_ok() {
// Restore the sender slot now that the sender is finished, and notify waiting senders.
self.channel.slot.replace_sender_slot(SenderSlot(receiver_slot.0.clone()));
self.channel.waiting_senders.notify_one();
}
// Restore the receiver slot and notify waiting receivers.
// trace!("receiver done, restoring slot");
self.channel.slot.replace_receiver_slot(receiver_slot);
self.channel.waiting_receivers.notify_one();
// trace!("rendezvous: receiver done, returning from receive().");
#[cfg(trace_channel)]
trace!("rendezvous: received msg: {:?}", debugit!(retval));
retval
}
/// Tries to receive a message, only succeeding if a sender is ready and waiting.
///
/// If the sender was not ready, it returns an error without blocking.
///
/// Note that if the non-blocking `try_send` and `try_receive` functions are only ever used,
/// then the message will never be delivered because the sender and receiver cannot possibly rendezvous.
pub fn try_receive(&self) -> Result<T, &'static str> {
unimplemented!()
}
}
// TODO: implement drop for sender and receiver in order to notify the other side of a disconnect