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rust/src/libsync/lock.rs

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// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Wrappers for safe, shared, mutable memory between tasks
//!
//! The wrappers in this module build on the primitives from `sync::raw` to
//! provide safe interfaces around using the primitive locks. These primitives
//! implement a technique called "poisoning" where when a task failed with a
//! held lock, all future attempts to use the lock will fail.
//!
//! For example, if two tasks are contending on a mutex and one of them fails
//! after grabbing the lock, the second task will immediately fail because the
//! lock is now poisoned.
use std::task;
use std::ty::Unsafe;
use raw;
/****************************************************************************
* Poisoning helpers
****************************************************************************/
struct PoisonOnFail<'a> {
flag: &'a mut bool,
failed: bool,
}
impl<'a> PoisonOnFail<'a> {
fn check(flag: bool, name: &str) {
if flag {
fail!("Poisoned {} - another task failed inside!", name);
}
}
fn new<'a>(flag: &'a mut bool, name: &str) -> PoisonOnFail<'a> {
PoisonOnFail::check(*flag, name);
PoisonOnFail {
flag: flag,
failed: task::failing()
}
}
}
#[unsafe_destructor]
impl<'a> Drop for PoisonOnFail<'a> {
fn drop(&mut self) {
if !self.failed && task::failing() {
*self.flag = true;
}
}
}
/****************************************************************************
* Condvar
****************************************************************************/
enum Inner<'a> {
InnerMutex(raw::MutexGuard<'a>),
InnerRWLock(raw::RWLockWriteGuard<'a>),
}
impl<'b> Inner<'b> {
fn cond<'a>(&'a self) -> &'a raw::Condvar<'b> {
match *self {
InnerMutex(ref m) => &m.cond,
InnerRWLock(ref m) => &m.cond,
}
}
}
/// A condition variable, a mechanism for unlock-and-descheduling and
/// signaling, for use with the lock types.
pub struct Condvar<'a> {
name: &'static str,
// n.b. Inner must be after PoisonOnFail because we must set the poison flag
// *inside* the mutex, and struct fields are destroyed top-to-bottom
// (destroy the lock guard last).
poison: PoisonOnFail<'a>,
inner: Inner<'a>,
}
impl<'a> Condvar<'a> {
/// Atomically exit the associated lock and block until a signal is sent.
///
/// wait() is equivalent to wait_on(0).
///
/// # Failure
///
/// A task which is killed while waiting on a condition variable will wake
/// up, fail, and unlock the associated lock as it unwinds.
#[inline]
pub fn wait(&self) { self.wait_on(0) }
/// Atomically exit the associated lock and block on a specified condvar
/// until a signal is sent on that same condvar.
///
/// The associated lock must have been initialised with an appropriate
/// number of condvars. The condvar_id must be between 0 and num_condvars-1
/// or else this call will fail.
#[inline]
pub fn wait_on(&self, condvar_id: uint) {
assert!(!*self.poison.flag);
self.inner.cond().wait_on(condvar_id);
// This is why we need to wrap sync::condvar.
PoisonOnFail::check(*self.poison.flag, self.name);
}
/// Wake up a blocked task. Returns false if there was no blocked task.
#[inline]
pub fn signal(&self) -> bool { self.signal_on(0) }
/// Wake up a blocked task on a specified condvar (as
/// sync::cond.signal_on). Returns false if there was no blocked task.
#[inline]
pub fn signal_on(&self, condvar_id: uint) -> bool {
assert!(!*self.poison.flag);
self.inner.cond().signal_on(condvar_id)
}
/// Wake up all blocked tasks. Returns the number of tasks woken.
#[inline]
pub fn broadcast(&self) -> uint { self.broadcast_on(0) }
/// Wake up all blocked tasks on a specified condvar (as
/// sync::cond.broadcast_on). Returns the number of tasks woken.
#[inline]
pub fn broadcast_on(&self, condvar_id: uint) -> uint {
assert!(!*self.poison.flag);
self.inner.cond().broadcast_on(condvar_id)
}
}
/****************************************************************************
* Mutex
****************************************************************************/
/// A wrapper type which provides synchronized access to the underlying data, of
/// type `T`. A mutex always provides exclusive access, and concurrent requests
/// will block while the mutex is already locked.
///
/// # Example
///
/// ```
/// use sync::{Mutex, Arc};
///
/// let mutex = Arc::new(Mutex::new(1));
/// let mutex2 = mutex.clone();
///
/// spawn(proc() {
/// let mut val = mutex2.lock();
/// *val += 1;
/// val.cond.signal();
/// });
///
/// let mut value = mutex.lock();
/// while *value != 2 {
/// value.cond.wait();
/// }
/// ```
pub struct Mutex<T> {
lock: raw::Mutex,
failed: Unsafe<bool>,
data: Unsafe<T>,
}
/// An guard which is created by locking a mutex. Through this guard the
/// underlying data can be accessed.
pub struct MutexGuard<'a, T> {
data: &'a mut T,
/// Inner condition variable connected to the locked mutex that this guard
/// was created from. This can be used for atomic-unlock-and-deschedule.
pub cond: Condvar<'a>,
}
impl<T: Send> Mutex<T> {
/// Creates a new mutex to protect the user-supplied data.
pub fn new(user_data: T) -> Mutex<T> {
Mutex::new_with_condvars(user_data, 1)
}
/// Create a new mutex, with a specified number of associated condvars.
///
/// This will allow calling wait_on/signal_on/broadcast_on with condvar IDs
/// between 0 and num_condvars-1. (If num_condvars is 0, lock_cond will be
/// allowed but any operations on the condvar will fail.)
pub fn new_with_condvars(user_data: T, num_condvars: uint) -> Mutex<T> {
Mutex {
lock: raw::Mutex::new_with_condvars(num_condvars),
failed: Unsafe::new(false),
data: Unsafe::new(user_data),
}
}
/// Access the underlying mutable data with mutual exclusion from other
/// tasks. The returned value is an RAII guard which will unlock the mutex
/// when dropped. All concurrent tasks attempting to lock the mutex will
/// block while the returned value is still alive.
///
/// # Failure
///
/// Failing while inside the Mutex will unlock the Mutex while unwinding, so
/// that other tasks won't block forever. It will also poison the Mutex:
/// any tasks that subsequently try to access it (including those already
/// blocked on the mutex) will also fail immediately.
#[inline]
pub fn lock<'a>(&'a self) -> MutexGuard<'a, T> {
let guard = self.lock.lock();
// These two accesses are safe because we're guranteed at this point
// that we have exclusive access to this mutex. We are indeed able to
// promote ourselves from &Mutex to `&mut T`
let poison = unsafe { &mut *self.failed.get() };
let data = unsafe { &mut *self.data.get() };
MutexGuard {
data: data,
cond: Condvar {
name: "Mutex",
poison: PoisonOnFail::new(poison, "Mutex"),
inner: InnerMutex(guard),
},
}
}
}
impl<'a, T: Send> Deref<T> for MutexGuard<'a, T> {
fn deref<'a>(&'a self) -> &'a T { &*self.data }
}
impl<'a, T: Send> DerefMut<T> for MutexGuard<'a, T> {
fn deref_mut<'a>(&'a mut self) -> &'a mut T { &mut *self.data }
}
/****************************************************************************
* R/W lock protected lock
****************************************************************************/
/// A dual-mode reader-writer lock. The data can be accessed mutably or
/// immutably, and immutably-accessing tasks may run concurrently.
///
/// # Example
///
/// ```
/// use sync::{RWLock, Arc};
///
/// let lock1 = Arc::new(RWLock::new(1));
/// let lock2 = lock1.clone();
///
/// spawn(proc() {
/// let mut val = lock2.write();
/// *val = 3;
/// let val = val.downgrade();
/// println!("{}", *val);
/// });
///
/// let val = lock1.read();
/// println!("{}", *val);
/// ```
pub struct RWLock<T> {
lock: raw::RWLock,
failed: Unsafe<bool>,
data: Unsafe<T>,
}
/// A guard which is created by locking an rwlock in write mode. Through this
/// guard the underlying data can be accessed.
pub struct RWLockWriteGuard<'a, T> {
data: &'a mut T,
/// Inner condition variable that can be used to sleep on the write mode of
/// this rwlock.
pub cond: Condvar<'a>,
}
/// A guard which is created by locking an rwlock in read mode. Through this
/// guard the underlying data can be accessed.
pub struct RWLockReadGuard<'a, T> {
data: &'a T,
guard: raw::RWLockReadGuard<'a>,
}
impl<T: Send + Share> RWLock<T> {
/// Create a reader/writer lock with the supplied data.
pub fn new(user_data: T) -> RWLock<T> {
RWLock::new_with_condvars(user_data, 1)
}
/// Create a reader/writer lock with the supplied data and a specified number
/// of condvars (as sync::RWLock::new_with_condvars).
pub fn new_with_condvars(user_data: T, num_condvars: uint) -> RWLock<T> {
RWLock {
lock: raw::RWLock::new_with_condvars(num_condvars),
failed: Unsafe::new(false),
data: Unsafe::new(user_data),
}
}
/// Access the underlying data mutably. Locks the rwlock in write mode;
/// other readers and writers will block.
///
/// # Failure
///
/// Failing while inside the lock will unlock the lock while unwinding, so
/// that other tasks won't block forever. As Mutex.lock, it will also poison
/// the lock, so subsequent readers and writers will both also fail.
#[inline]
pub fn write<'a>(&'a self) -> RWLockWriteGuard<'a, T> {
let guard = self.lock.write();
// These two accesses are safe because we're guranteed at this point
// that we have exclusive access to this rwlock. We are indeed able to
// promote ourselves from &RWLock to `&mut T`
let poison = unsafe { &mut *self.failed.get() };
let data = unsafe { &mut *self.data.get() };
RWLockWriteGuard {
data: data,
cond: Condvar {
name: "RWLock",
poison: PoisonOnFail::new(poison, "RWLock"),
inner: InnerRWLock(guard),
},
}
}
/// Access the underlying data immutably. May run concurrently with other
/// reading tasks.
///
/// # Failure
///
/// Failing will unlock the lock while unwinding. However, unlike all other
/// access modes, this will not poison the lock.
pub fn read<'a>(&'a self) -> RWLockReadGuard<'a, T> {
let guard = self.lock.read();
PoisonOnFail::check(unsafe { *self.failed.get() }, "RWLock");
RWLockReadGuard {
guard: guard,
data: unsafe { &*self.data.get() },
}
}
}
impl<'a, T: Send + Share> RWLockWriteGuard<'a, T> {
/// Consumes this write lock token, returning a new read lock token.
///
/// This will allow pending readers to come into the lock.
pub fn downgrade(self) -> RWLockReadGuard<'a, T> {
let RWLockWriteGuard { data, cond } = self;
// convert the data to read-only explicitly
let data = &*data;
let guard = match cond.inner {
InnerMutex(..) => unreachable!(),
InnerRWLock(guard) => guard.downgrade()
};
RWLockReadGuard { guard: guard, data: data }
}
}
impl<'a, T: Send + Share> Deref<T> for RWLockReadGuard<'a, T> {
fn deref<'a>(&'a self) -> &'a T { self.data }
}
impl<'a, T: Send + Share> Deref<T> for RWLockWriteGuard<'a, T> {
fn deref<'a>(&'a self) -> &'a T { &*self.data }
}
impl<'a, T: Send + Share> DerefMut<T> for RWLockWriteGuard<'a, T> {
fn deref_mut<'a>(&'a mut self) -> &'a mut T { &mut *self.data }
}
/****************************************************************************
* Barrier
****************************************************************************/
/// A barrier enables multiple tasks to synchronize the beginning
/// of some computation.
///
/// ```rust
/// use sync::{Arc, Barrier};
///
/// let barrier = Arc::new(Barrier::new(10));
/// for _ in range(0, 10) {
/// let c = barrier.clone();
/// // The same messages will be printed together.
/// // You will NOT see any interleaving.
/// spawn(proc() {
/// println!("before wait");
/// c.wait();
/// println!("after wait");
/// });
/// }
/// ```
pub struct Barrier {
lock: Mutex<BarrierState>,
num_tasks: uint,
}
// The inner state of a double barrier
struct BarrierState {
count: uint,
generation_id: uint,
}
impl Barrier {
/// Create a new barrier that can block a given number of tasks.
pub fn new(num_tasks: uint) -> Barrier {
Barrier {
lock: Mutex::new(BarrierState {
count: 0,
generation_id: 0,
}),
num_tasks: num_tasks,
}
}
/// Block the current task until a certain number of tasks is waiting.
pub fn wait(&self) {
let mut lock = self.lock.lock();
let local_gen = lock.generation_id;
lock.count += 1;
if lock.count < self.num_tasks {
// We need a while loop to guard against spurious wakeups.
// http://en.wikipedia.org/wiki/Spurious_wakeup
while local_gen == lock.generation_id &&
lock.count < self.num_tasks {
lock.cond.wait();
}
} else {
lock.count = 0;
lock.generation_id += 1;
lock.cond.broadcast();
}
}
}
/****************************************************************************
* Tests
****************************************************************************/
#[cfg(test)]
mod tests {
use std::comm::Empty;
use std::task;
use arc::Arc;
use super::{Mutex, Barrier, RWLock};
#[test]
fn test_mutex_arc_condvar() {
let arc = Arc::new(Mutex::new(false));
let arc2 = arc.clone();
let (tx, rx) = channel();
task::spawn(proc() {
// wait until parent gets in
rx.recv();
let mut lock = arc2.lock();
*lock = true;
lock.cond.signal();
});
let lock = arc.lock();
tx.send(());
assert!(!*lock);
while !*lock {
lock.cond.wait();
}
}
#[test] #[should_fail]
fn test_arc_condvar_poison() {
let arc = Arc::new(Mutex::new(1));
let arc2 = arc.clone();
let (tx, rx) = channel();
spawn(proc() {
rx.recv();
let lock = arc2.lock();
lock.cond.signal();
// Parent should fail when it wakes up.
fail!();
});
let lock = arc.lock();
tx.send(());
while *lock == 1 {
lock.cond.wait();
}
}
#[test] #[should_fail]
fn test_mutex_arc_poison() {
let arc = Arc::new(Mutex::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.lock();
assert_eq!(*lock, 2);
});
let lock = arc.lock();
assert_eq!(*lock, 1);
}
#[test]
fn test_mutex_arc_nested() {
// Tests nested mutexes and access
// to underlaying data.
let arc = Arc::new(Mutex::new(1));
let arc2 = Arc::new(Mutex::new(arc));
task::spawn(proc() {
let lock = arc2.lock();
let lock2 = lock.deref().lock();
assert_eq!(*lock2, 1);
});
}
#[test]
fn test_mutex_arc_access_in_unwind() {
let arc = Arc::new(Mutex::new(1i));
let arc2 = arc.clone();
let _ = task::try::<()>(proc() {
struct Unwinder {
i: Arc<Mutex<int>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
let mut lock = self.i.lock();
*lock += 1;
}
}
let _u = Unwinder { i: arc2 };
fail!();
});
let lock = arc.lock();
assert_eq!(*lock, 2);
}
#[test] #[should_fail]
fn test_rw_arc_poison_wr() {
let arc = Arc::new(RWLock::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.write();
assert_eq!(*lock, 2);
});
let lock = arc.read();
assert_eq!(*lock, 1);
}
#[test] #[should_fail]
fn test_rw_arc_poison_ww() {
let arc = Arc::new(RWLock::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.write();
assert_eq!(*lock, 2);
});
let lock = arc.write();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_rr() {
let arc = Arc::new(RWLock::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.read();
assert_eq!(*lock, 2);
});
let lock = arc.read();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_rw() {
let arc = Arc::new(RWLock::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.read();
assert_eq!(*lock, 2);
});
let lock = arc.write();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_dr() {
let arc = Arc::new(RWLock::new(1));
let arc2 = arc.clone();
let _ = task::try(proc() {
let lock = arc2.write().downgrade();
assert_eq!(*lock, 2);
});
let lock = arc.write();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc() {
let arc = Arc::new(RWLock::new(0));
let arc2 = arc.clone();
let (tx, rx) = channel();
task::spawn(proc() {
let mut lock = arc2.write();
for _ in range(0, 10) {
let tmp = *lock;
*lock = -1;
task::deschedule();
*lock = tmp + 1;
}
tx.send(());
});
// Readers try to catch the writer in the act
let mut children = Vec::new();
for _ in range(0, 5) {
let arc3 = arc.clone();
let mut builder = task::task();
children.push(builder.future_result());
builder.spawn(proc() {
let lock = arc3.read();
assert!(*lock >= 0);
});
}
// Wait for children to pass their asserts
for r in children.mut_iter() {
assert!(r.recv().is_ok());
}
// Wait for writer to finish
rx.recv();
let lock = arc.read();
assert_eq!(*lock, 10);
}
#[test]
fn test_rw_arc_access_in_unwind() {
let arc = Arc::new(RWLock::new(1i));
let arc2 = arc.clone();
let _ = task::try::<()>(proc() {
struct Unwinder {
i: Arc<RWLock<int>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
let mut lock = self.i.write();
*lock += 1;
}
}
let _u = Unwinder { i: arc2 };
fail!();
});
let lock = arc.read();
assert_eq!(*lock, 2);
}
#[test]
fn test_rw_downgrade() {
// (1) A downgrader gets in write mode and does cond.wait.
// (2) A writer gets in write mode, sets state to 42, and does signal.
// (3) Downgrader wakes, sets state to 31337.
// (4) tells writer and all other readers to contend as it downgrades.
// (5) Writer attempts to set state back to 42, while downgraded task
// and all reader tasks assert that it's 31337.
let arc = Arc::new(RWLock::new(0));
// Reader tasks
let mut reader_convos = Vec::new();
for _ in range(0, 10) {
let ((tx1, rx1), (tx2, rx2)) = (channel(), channel());
reader_convos.push((tx1, rx2));
let arcn = arc.clone();
task::spawn(proc() {
rx1.recv(); // wait for downgrader to give go-ahead
let lock = arcn.read();
assert_eq!(*lock, 31337);
tx2.send(());
});
}
// Writer task
let arc2 = arc.clone();
let ((tx1, rx1), (tx2, rx2)) = (channel(), channel());
task::spawn(proc() {
rx1.recv();
{
let mut lock = arc2.write();
assert_eq!(*lock, 0);
*lock = 42;
lock.cond.signal();
}
rx1.recv();
{
let mut lock = arc2.write();
// This shouldn't happen until after the downgrade read
// section, and all other readers, finish.
assert_eq!(*lock, 31337);
*lock = 42;
}
tx2.send(());
});
// Downgrader (us)
let mut lock = arc.write();
tx1.send(()); // send to another writer who will wake us up
while *lock == 0 {
lock.cond.wait();
}
assert_eq!(*lock, 42);
*lock = 31337;
// send to other readers
for &(ref mut rc, _) in reader_convos.mut_iter() {
rc.send(())
}
let lock = lock.downgrade();
// complete handshake with other readers
for &(_, ref mut rp) in reader_convos.mut_iter() {
rp.recv()
}
tx1.send(()); // tell writer to try again
assert_eq!(*lock, 31337);
drop(lock);
rx2.recv(); // complete handshake with writer
}
#[cfg(test)]
fn test_rw_write_cond_downgrade_read_race_helper() {
// Tests that when a downgrader hands off the "reader cloud" lock
// because of a contending reader, a writer can't race to get it
// instead, which would result in readers_and_writers. This tests
// the raw module rather than this one, but it's here because an
// rwarc gives us extra shared state to help check for the race.
let x = Arc::new(RWLock::new(true));
let (tx, rx) = channel();
// writer task
let xw = x.clone();
task::spawn(proc() {
let mut lock = xw.write();
tx.send(()); // tell downgrader it's ok to go
lock.cond.wait();
// The core of the test is here: the condvar reacquire path
// must involve order_lock, so that it cannot race with a reader
// trying to receive the "reader cloud lock hand-off".
*lock = false;
});
rx.recv(); // wait for writer to get in
let lock = x.write();
assert!(*lock);
// make writer contend in the cond-reacquire path
lock.cond.signal();
// make a reader task to trigger the "reader cloud lock" handoff
let xr = x.clone();
let (tx, rx) = channel();
task::spawn(proc() {
tx.send(());
drop(xr.read());
});
rx.recv(); // wait for reader task to exist
let lock = lock.downgrade();
// if writer mistakenly got in, make sure it mutates state
// before we assert on it
for _ in range(0, 5) { task::deschedule(); }
// make sure writer didn't get in.
assert!(*lock);
}
#[test]
fn test_rw_write_cond_downgrade_read_race() {
// Ideally the above test case would have deschedule statements in it
// that helped to expose the race nearly 100% of the time... but adding
// deschedules in the intuitively-right locations made it even less
// likely, and I wasn't sure why :( . This is a mediocre "next best"
// option.
for _ in range(0, 8) {
test_rw_write_cond_downgrade_read_race_helper();
}
}
/************************************************************************
* Barrier tests
************************************************************************/
#[test]
fn test_barrier() {
let barrier = Arc::new(Barrier::new(10));
let (tx, rx) = channel();
for _ in range(0, 9) {
let c = barrier.clone();
let tx = tx.clone();
spawn(proc() {
c.wait();
tx.send(true);
});
}
// At this point, all spawned tasks should be blocked,
// so we shouldn't get anything from the port
assert!(match rx.try_recv() {
Empty => true,
_ => false,
});
barrier.wait();
// Now, the barrier is cleared and we should get data.
for _ in range(0, 9) {
rx.recv();
}
}
}
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