Rust Synchronization Primitives

Rust - Synchronization and Concurrency
Safe synchronization abstractions and their implementation

Corey (Rust) Richardson

February 17, 2014

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

Concurrency model

”Tasks” as unit of computation
No observable shared memory
No race conditions! †

No race conditions?

How can we avoid race conditions?
A type system which enables safe sharing of data
Careful design of concurrency abstractions

Hello World

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

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
}

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 })
}
}

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
}
}
}

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 };
}
}
}

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() }
}
}

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)
}
}

Mutexes!

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

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>
}

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> { ... }
}

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
}
}

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
}

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
}
}

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
}

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));
}

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();
}
}
}

Releasing a Semaphore (V)

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

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)
}
}

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.

Thank You

Thanks for your time!

Rust Synchronization Primitives

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