These are slides for a presentation I gave to my OS class. It was mostly me talking, using the code to drive the examples and show how Rust approaches the problems of synchronization. Not super detailed, doesn't get into the meaty stuff (Condvar, mutex::Mutex, etc)

1. Rust - Synchronization and Concurrency
Safe synchronization abstractions and their implementation
Corey (Rust) Richardson
February 17, 2014

2. What is Rust?
Rust is a systems language, aimed at replacing C++, with the
following design goals, in roughly descending order of importance:
Zero-cost abstraction
Easy, safe concurrency and parallelism
Memory safety (no data races)
Type safety (no willy-nilly casting)
Simplicity
Compilation speed

3. Concurrency model
”Tasks” as unit of computation
No observable shared memory
No race conditions! †

4. No race conditions?
How can we avoid race conditions?
A type system which enables safe sharing of data
Careful design of concurrency abstractions

5. Hello World
fn main() {
println("Hello, world!");
}

6. UnsafeArc
Unsafe data structure. Provides atomic reference counting of a
type. Ensures memory does not leak.
pub struct UnsafeArc<T> {
priv data: *mut ArcData<T>
}
struct ArcData<T> {
count: AtomicUint,
data: T
}

7. UnsafeArc cont.
fn new_inner<T>(data: T, initial_count: uint) -> *mut ArcData<T>
let data = box ArcData {
count: AtomicUint::new(initial_count),
data: data
}
cast::transmute(data)
}
impl<T: Send> UnsafeArc<T> {
pub fn new(data: T) -> UnsafeArc<T> {
unsafe { UnsafeArc { data: new_inner(data, 1) } }
}
pub fn new2(data: T) -> (UnsafeArc<T>, UnsafeArc<T>) {
unsafe {
let ptr = new_inner(data, 2);
(UnsafeArc { data: ptr }, UnsafeArc { data: ptr })
}
}

8. UnsafeArc cont.
pub fn get(&self) -> *mut T {
unsafe {
// problems?
assert!((*self.data).count.load(Relaxed) > 0);
return &mut (*self.data).data as *mut T;
}
}
pub fn get_immut(&self) -> *T {
unsafe {
// problems?
assert!((*self.data).count.load(Relaxed) > 0);
return &(*self.data).data as *T;
}
}
pub fn is_owned(&self) -> bool {
unsafe {
// problems?
(*self.data).count.load(Relaxed) == 1
}
}
}

9. UnsafeArc cloning
impl<T: Send> Clone for UnsafeArc<T> {
fn clone(&self) -> UnsafeArc<T> {
unsafe {
let old_count =
(*self.data).count
.fetch_add(1, Acquire);
//
^~~~~~~ Why?
assert!(old_count >= 1);
return UnsafeArc { data: self.data };
}
}
}

10. Adding Safety
Arc: wraps UnsafeArc, provides read-only access.
pub struct Arc<T> { priv x: UnsafeArc<T> }
impl<T: Freeze + Send> Arc<T> {
pub fn new(data: T) -> Arc<T> {
Arc { x: UnsafeArc::new(data) }
}
pub fn get<’a>(&’a self) -> &’a T {
unsafe { &*self.x.get_immut() }
}
}

11. Mutexes?
pub struct Mutex { priv sem: Sem<~[WaitQueue]> }
impl Mutex {
pub fn new() -> Mutex { Mutex::new_with_condvars(1) }
pub fn new_with_condvars(num: uint) -> Mutex {
Mutex { sem: Sem::new_and_signal(1, num) }
}
pub fn lock<U>(&self, blk: || -> U) -> U {
// magic?
(&self.sem).access(blk)
}
pub fn lock_cond<U>(&self,
blk: |c: &Condvar| -> U) -> U {
(&self.sem).access_cond(blk)
}
}

12. Mutexes!
Mutexes in Rust are implemented on top of semaphores, using 100
No ‘unlock’ operation? Closures!

13. Wait Queues
Wait queues provide an ordering when waiting on a lock.
// Each waiting task receives on one of these.
type WaitEnd = Port<()>;
type SignalEnd = Chan<()>;
// A doubly-ended queue of waiting tasks.
struct WaitQueue {
head: Port<SignalEnd>,
tail: Chan<SignalEnd>
}

14. Channels and Ports
Message passing. Provides a way to send ‘Send‘ data to another
task. Very efficient, single-reader, single-writer.
impl <T: Send> Chan<T> {
fn send(&self, data: T) { ... }
fn try_send(&self, data: T) -> bool { ... }
}
impl <T: Send> Port<T> {
fn recv(&self) -> T { ... }
fn try_recv(&self) -> TryRecvResult<T> { ... }
}

15. Wait Queue Implementation
Given Ports and Chans, how can we express wait queues?
impl WaitQueue {
fn signal(&self) -> bool {
match self.head.try_recv() {
comm::Data(ch) => {
// Send a wakeup signal. If the waiter
// was killed, its port will
// have closed. Keep trying until we
// get a live task.
if ch.try_send(()) {
true
} else {
self.signal()
}
}
_ => false
}
}

16. Wait Queue Impl Cont.
fn broadcast(&self) -> uint {
let mut count = 0;
loop {
match self.head.try_recv() {
comm::Data(ch) => {
if ch.try_send(()) {
count += 1;
}
}
_ => break
}
}
count
}

17. Wait Queue Impl End
fn wait_end(&self) -> WaitEnd {
let (wait_end, signal_end) = Chan::new();
assert!(self.tail.try_send(signal_end));
wait_end
}
}

18. Raw Semaphores
We have a way to express order and waiting, now to build some
actual *synchronization*.
struct Sem<Q>(UnsafeArc<SemInner<Q>>);
struct SemInner<Q> {
lock: LowLevelMutex,
count: int,
waiters:
WaitQueue,
// Can be either unit or another waitqueue.
// Some sems shouldn’t come with
// a condition variable attached, others should.
blocked:
Q
}

19. Semaphore Implementation
impl<Q: Send> Sem<Q> {
pub fn access<U>(&self, blk: || -> U) -> U {
(|| {
self.acquire();
blk()
}).finally(|| {
self.release();
})
}
unsafe fn with(&self, f: |&mut SemInner<Q>|) {
let Sem(ref arc) = *self;
let state = arc.get();
let _g = (*state).lock.lock();
// unlock????
f(cast::transmute(state));
}

20. Acquiring a semaphore (P)
pub fn acquire(&self) {
unsafe {
let mut waiter_nobe = None;
self.with(|state| {
state.count -= 1;
if state.count < 0 {
// Create waiter nobe, enqueue ourself,
// outer scope we need to block.
waiter_nobe = Some(state.waiters.wait_e
}
});
// Need to wait outside the exclusive.
if waiter_nobe.is_some() {
let _ = waiter_nobe.unwrap().recv();
}
}
}

21. Releasing a Semaphore (V)
pub fn release(&self) {
unsafe {
self.with(|state| {
state.count += 1;
if state.count <= 0 {
state.waiters.signal();
}
})
}
}
}

22. Filling in the last pieces
impl Sem<~[WaitQueue]> {
fn new_and_signal(count: int, num_condvars: uint) -> Se
let mut queues = ~[];
for _ in range(0, num_condvars) { queues.push(WaitQ
Sem::new(count, queues)
}
}

23. And more?
On top of these primitives, as we have seen in class, every other
synchronization primitive can be constructed. In particular, we also
provide starvation-free Reader-Writer locks, Barriers, and
Copy-on-Write Arcs.

24. Thank You
Thanks for your time!