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#![allow(unused_imports)]
use super::shared::{fence_acquire, invalid_mut, AtomicPtrRmw, SpinWait, StrictProvenance, Waiter};
use std::{
fmt,
pin::Pin,
ptr::{self, NonNull},
sync::atomic::{AtomicPtr, Ordering},
};
const UNLOCKED: usize = 0;
const LOCKED: usize = 1;
const READING: usize = 2;
const QUEUED: usize = 4;
const QUEUE_LOCKED: usize = 8;
const READER_SHIFT: u32 = 16usize.trailing_zeros();
const SINGLE_READER: usize = LOCKED | READING | (1 << READER_SHIFT);
/// Raw rwlock type implemented with lock-free userspace thread queues.
#[derive(Default)]
#[repr(transparent)]
pub struct RawRwLock {
/// This atomic integer holds the current state of the rwlock instance.
/// The four least significant bits are used to track the different states of the RwLock.
///
/// # State table:
///
/// LOCKED | READING | QUEUED | QUEUE_LOCKED | Remaining | Description
/// 0 | 0 | 0 | 0 | 0 | The RwLock is unlocked and in an empty state.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 0 | 0 | 0 | 0 | One writer holds the lock and there are no waiting threads.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 0 | 1 | 0 | *Waiter | One writer holds the lock and the Remaining bits point to
/// | | | | | the head Waiter node of the waiting-thread queue.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 0 | 1 | 1 | *Waiter | One writer holds the lock and the Remaining bits point to
/// | | | | | the head Waiter node of the waiting thread queue. There is
/// | | | | | also a thread which is updating the waiting-thread queue.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 0 | 0 | 1 | 1 | *Waiter | The rwlock is not held, but there are waiting threads.
/// | | | | | There is also one thread which is trying to dequeue and
/// | | | | | wake up a thread from the waiting-thread queue.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 1 | 0 | 0 | n | `n` readers hold the lock and there are no waiting threads.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 1 | 0 | 1 | *Waiter | The lock is held by readers and the Remaining bits point to
/// | | | | | the head Waiter node of the waiting thread queue. The reader
/// | | | | | count has also been moved to the `counter` field on the tail
/// | | | | | node of the waiting-thread queue.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
/// 1 | 1 | 1 | 1 | *Waiter | The lock is held by readers and the remaining bits point to
/// | | | | | the head Waiter node of the waiting thread queue. There is
/// | | | | | also a thread which is updating the waiting-thread queue.
/// -------+---------+--------+--------------+-----------+-------------------------------------------------------------
pub(super) state: AtomicPtr<Waiter>,
}
impl fmt::Debug for RawRwLock {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("RawRwLock { .. }")
}
}
unsafe impl Send for RawRwLock {}
unsafe impl Sync for RawRwLock {}
unsafe impl lock_api::RawRwLock for RawRwLock {
type GuardMarker = crate::GuardMarker;
const INIT: Self = Self {
state: AtomicPtr::new(invalid_mut(UNLOCKED)),
};
#[inline]
fn is_locked(&self) -> bool {
let state = self.state.load(Ordering::Relaxed);
state.address() & LOCKED != 0
}
#[inline]
fn is_locked_exclusive(&self) -> bool {
let state = self.state.load(Ordering::Relaxed);
state.address() & (LOCKED | READING) == LOCKED
}
#[inline]
fn try_lock_exclusive(&self) -> bool {
self.try_lock_exclusive_fast()
}
#[inline]
fn lock_exclusive(&self) {
if !self.try_lock_exclusive() {
self.lock_exclusive_slow();
}
}
#[inline]
unsafe fn unlock_exclusive(&self) {
self.unlock_exclusive_fast()
}
#[inline]
fn try_lock_shared(&self) -> bool {
self.try_lock_shared_fast() || self.try_lock_shared_slow()
}
#[inline]
fn lock_shared(&self) {
if !self.try_lock_shared_fast() {
self.lock_shared_slow();
}
}
#[inline]
unsafe fn unlock_shared(&self) {
if !self.unlock_shared_fast() {
self.unlock_shared_slow();
}
}
}
// --- X86 Specializations
#[cfg(all(any(target_arch = "x86", target_arch = "x86_64"), not(miri)))]
impl RawRwLock {
#[inline(always)]
fn try_lock_exclusive_assuming(&self, _state: *mut Waiter) -> bool {
use lock_api::RawRwLock as _;
self.try_lock_exclusive()
}
#[inline(always)]
fn try_lock_exclusive_fast(&self) -> bool {
// On x86, `lock bts` is often faster for acquiring exclusive ownership
// than a `lock cmpxchg` as the former wont spuriously fail when a thread
// is updating the QUEUE_LOCKED bit or adding themselves to the queue.
unsafe {
let mut old_locked_bit: u8;
#[cfg(target_pointer_width = "64")]
std::arch::asm!(
"lock bts qword ptr [{0:r}], 0",
"setc {1}",
in(reg) &self.state,
out(reg_byte) old_locked_bit,
options(nostack),
);
#[cfg(target_pointer_width = "32")]
std::arch::asm!(
"lock bts dword ptr [{0:e}], 0",
"setc {1}",
in(reg) &self.state,
out(reg_byte) old_locked_bit,
options(nostack),
);
let acquired = old_locked_bit == 0;
if acquired {
fence_acquire(&self.state);
}
acquired
}
}
#[inline(always)]
unsafe fn unlock_exclusive_fast(&self) {
// On x86, we unlock the exclusive lock first, then try and wake later.
// This is faster than using a `lock cmpxchg` loop as it doesn't have
// to fail and retry from other threads updating QUEUE_LOCKED bit or queueing themselves.
let locked = ptr::null_mut::<Waiter>().with_address(LOCKED);
let state = self.state.fetch_sub(locked, Ordering::Release);
debug_assert_eq!(state.address() & (LOCKED | READING), LOCKED);
// Only try to unpark if there's no QUEUE_LOCKED owner yet and if there's threads queued.
if state.address() & (QUEUED | QUEUE_LOCKED) == QUEUED {
self.try_unpark();
}
}
#[cold]
unsafe fn unlock_shared_and_unpark(&self) {
// On x86, we unlock the shared lock first, then try and wake later.
// This is faster than using a `lock cmpxchg` loop as it doesn't have
// to fail and retry from other threads updating QUEUE_LOCKED bit or queueing themselves.
let read_locked = ptr::null_mut::<Waiter>().with_address(LOCKED | READING);
let state = self.state.fetch_sub(read_locked, Ordering::Release);
debug_assert_eq!(state.address() & (LOCKED | READING), LOCKED | READING);
// Only try to unpark if there's no QUEUE_LOCKED owner yet and if there's threads queued.
if state.address() & (QUEUED | QUEUE_LOCKED) == QUEUED {
self.try_unpark();
}
}
#[cold]
fn try_unpark(&self) {
let mut state = self.state.load(Ordering::Relaxed);
// Try to grab the QUEUE_LOCKED bit to wake up threads iff:
// - theres no lock holder, as they can be the ones to do the wake up
// - there are still threads queued to actually wake up
// - the QUEUE_LOCKED bit isnt held as someone is already doing wake up
while state.address() & (LOCKED | QUEUED | QUEUE_LOCKED) == QUEUED {
let new_state = state.map_address(|addr| addr | QUEUE_LOCKED);
match self.state.compare_exchange_weak(
state,
new_state,
Ordering::Relaxed,
Ordering::Relaxed,
) {
Ok(_) => return unsafe { self.unpark(new_state) },
Err(e) => state = e,
}
}
}
}
#[cfg(any(miri, not(any(target_arch = "x86", target_arch = "x86_64"))))]
impl RawRwLock {
#[inline(always)]
fn try_lock_exclusive_assuming(&self, mut state: *mut Waiter) -> bool {
while state.address() & LOCKED == 0 {
match self.state.compare_exchange_weak(
state,
state.map_address(|addr| addr | LOCKED),
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => return true,
Err(e) => state = e,
}
}
false
}
#[inline(always)]
fn try_lock_exclusive_fast(&self) -> bool {
self.state
.compare_exchange(
invalid_mut(UNLOCKED),
invalid_mut(LOCKED),
Ordering::Acquire,
Ordering::Relaxed,
)
.is_ok()
}
#[inline(always)]
unsafe fn unlock_exclusive_fast(&self) {
if self
.state
.compare_exchange(
invalid_mut(LOCKED),
invalid_mut(UNLOCKED),
Ordering::Release,
Ordering::Relaxed,
)
.is_err()
{
self.unlock_and_unpark();
}
}
#[inline(always)]
unsafe fn unlock_shared_and_unpark(&self) {
self.unlock_and_unpark()
}
#[cold]
unsafe fn unlock_and_unpark(&self) {
let mut state = self.state.load(Ordering::Relaxed);
loop {
assert_ne!(state.address() & LOCKED, 0);
assert_ne!(state.address() & QUEUED, 0);
// Unlocks the rwlock and tries to grab the QUEUE_LOCKED bit for wake up.
let new_state = state.map_address(|mut addr| {
addr &= !(LOCKED | READING);
addr |= QUEUE_LOCKED;
addr
});
if let Err(e) = self.state.compare_exchange_weak(
state,
new_state,
Ordering::Release,
Ordering::Relaxed,
) {
state = e;
continue;
}
if state.address() & QUEUE_LOCKED == 0 {
self.unpark(new_state);
}
return;
}
}
}
// --- Generic Code
impl RawRwLock {
#[inline(always)]
fn try_lock_shared_assuming(
&self,
state: *mut Waiter,
) -> Option<Result<*mut Waiter, *mut Waiter>> {
// Returns None if the lock is held by a writer
if state.address() != UNLOCKED {
if state.address() & (LOCKED | READING | QUEUED) != (LOCKED | READING) {
return None;
}
}
// Check for reader count overflow when trying to add a reader.
// On overflow, readers will queue themselves and be woken up by the last active reader.
// Overflow is very unlikely though as it requires `usize::MAX >> 4` active readers at once.
// On a system where `usize` is 64 bits, that's over a quintillion (1 million ^ 5) readers.
// On a system where `usize` is 32 bits, that's still over 260 million readers.
if let Some(with_reader) = state.address().checked_add(1 << READER_SHIFT) {
return Some(self.state.compare_exchange_weak(
state,
state.with_address(with_reader | LOCKED | READING),
Ordering::Acquire,
Ordering::Relaxed,
));
}
None
}
#[inline(always)]
fn try_lock_shared_fast(&self) -> bool {
let state = self.state.load(Ordering::Relaxed);
let result = self.try_lock_shared_assuming(state);
matches!(result, Some(Ok(_)))
}
#[cold]
fn try_lock_shared_slow(&self) -> bool {
let mut state = self.state.load(Ordering::Relaxed);
loop {
match self.try_lock_shared_assuming(state) {
None => return false,
Some(Err(e)) => state = e,
Some(Ok(_)) => return true,
}
}
}
#[inline(always)]
unsafe fn unlock_shared_fast(&self) -> bool {
// Just go to the slow path if we're not the only reader
let state = self.state.load(Ordering::Relaxed);
if state.address() != SINGLE_READER {
return false;
}
self.state
.compare_exchange(
state.with_address(SINGLE_READER),
state.with_address(UNLOCKED),
Ordering::Release,
Ordering::Relaxed,
)
.is_ok()
}
#[cold]
unsafe fn unlock_shared_slow(&self) {
// Try to just bump the reader count down when there's no waiting threads.
// This only works because the Remaining bits still point to the reader count.
// When threads start waiting, they override these bits with the queue pointer.
let mut state = self.state.load(Ordering::Relaxed);
while state.address() & QUEUED == 0 {
assert_ne!(state.address() & LOCKED, 0);
assert_ne!(state.address() & READING, 0);
assert_ne!(state.address() >> READER_SHIFT, 0);
let mut new_state = state.map_address(|addr| addr - (1 << READER_SHIFT));
if state.address() == SINGLE_READER {
new_state = state.with_address(UNLOCKED);
}
match self.state.compare_exchange_weak(
state,
new_state,
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => return,
Err(e) => state = e,
}
}
// The'ers threads waiting on the RwLock.
// The reader count has moved to the tail of the queue.
assert_ne!(state.address() & LOCKED, 0);
assert_ne!(state.address() & QUEUED, 0);
assert_ne!(state.address() & READING, 0);
// Find the tail of the wait queue while also caching it at the current head.
// As long as the Waiter writes are atomic, this can be soundly racing with
// other callers to get_and_link_queue() like link_queue_or_unpark() or other readers.
// Acquire barrier to ensure Waiter queue writes to head happen before we start scanning.
fence_acquire(&self.state);
let (_head, tail) = Waiter::get_and_link_queue(state, |_| {});
// Decrement the reader count which was moved to the tail.
// Release barrier to ensure RwLock-protected reads/loads happen before we "release" the read lock.
let readers = tail.as_ref().counter.fetch_sub(1, Ordering::Release);
assert_ne!(readers, 0);
// The last reader unsets the LOCKED bit and tries to wake up waiting threads.
// Acquire barrier synchronizes with the Release to counter above to ensure
// that the unsetting of the LOCKED bit happens after all the readers reads/loads occur.
if readers == 1 {
fence_acquire(&self.state);
self.unlock_shared_and_unpark();
}
}
#[cold]
fn lock_exclusive_slow(&self) {
let is_writer = true;
let try_lock = |state: *mut Waiter| -> Option<bool> {
match state.address() & LOCKED {
0 => Some(self.try_lock_exclusive_assuming(state)),
_ => None,
}
};
self.lock_common(is_writer, try_lock)
}
#[cold]
fn lock_shared_slow(&self) {
let is_writer = false;
let try_lock = |state: *mut Waiter| -> Option<bool> {
let result = self.try_lock_shared_assuming(state)?;
Some(result.is_ok())
};
self.lock_common(is_writer, try_lock)
}
fn lock_common(&self, is_writer: bool, mut try_lock: impl FnMut(*mut Waiter) -> Option<bool>) {
Waiter::with(|waiter| {
waiter.waiting_on.set(Some(NonNull::from(self).cast()));
waiter.flags.set(is_writer as usize);
let mut spin = SpinWait::default();
loop {
let mut state = self.state.load(Ordering::Relaxed);
loop {
// Try to acquire the RwLock.
// On failure, spins a bit to decrease cache-line contension.
let mut backoff = SpinWait::default();
while let Some(was_locked) = try_lock(state) {
if was_locked {
return;
}
backoff.yield_now();
state = self.state.load(Ordering::Relaxed);
}
// We can't acquire the RwLock at the moment.
// Try to spin for a little in hopes the RwLock is released quickly.
// Also don't spin if threads are waiting as we should start waiting too.
if (state.address() & QUEUED == 0) && spin.try_yield_now() {
state = self.state.load(Ordering::Relaxed);
continue;
}
if unsafe { self.try_queue(&mut state, waiter.as_ref()) } {
assert!(waiter.parker.park(None));
break;
}
}
}
});
}
#[cold]
pub(super) unsafe fn try_requeue(&self, waiter: Pin<&Waiter>) -> bool {
let is_writer = waiter.flags.get() != 0;
assert!(is_writer);
let waiting_on = waiter.waiting_on.get();
assert_eq!(waiting_on, Some(NonNull::from(self).cast()));
let mut state = self.state.load(Ordering::Relaxed);
loop {
// Don't requeue if the waiter (which is a writer) could acquire the lock.
if state.address() & LOCKED == 0 {
return false;
}
// Returns true when this waiter is requeued
if self.try_queue(&mut state, waiter.as_ref()) {
return true;
}
}
}
unsafe fn try_queue(&self, state: &mut *mut Waiter, waiter: Pin<&Waiter>) -> bool {
// Prepare to push our waiter to the head of the wait queue.
let waiter_ptr = NonNull::from(&*waiter).as_ptr();
let mut new_state = waiter_ptr.map_address(|addr| {
let state_bits = (*state).address() & !Waiter::MASK;
addr | state_bits | QUEUED
});
if (*state).address() & QUEUED == 0 {
// The first queued waiter will be the tail and it now needs to
// track the readers since its overriding the remaining state bits.
let readers = (*state).address() >> READER_SHIFT;
waiter.counter.store(readers, Ordering::Relaxed);
// The first queued waiter also sets its `tail` field to itself.
// This allows `Waiter::get_and_link_queue` to eventually find the queue tail.
waiter.prev.set(None);
waiter.next.set(None);
waiter.tail.set(Some(NonNull::from(&*waiter)));
} else {
// The thread which holds the QUEUE_LOCKED bit, or active read-lock holders, will update the queue.
// Since there's multiple waiting threads now, try to grab the QUEUE_LOCKED bit in order to update the queue.
let head = NonNull::new((*state).map_address(|addr| addr & Waiter::MASK));
new_state = new_state.map_address(|addr| addr | QUEUE_LOCKED);
// Other waiters will link themselves onto the waiter queue in a stack-like fashion;
// Leaving the `tail` field unset for Waiter::get_and_link_queue() to traverse and cache the found tail.
waiter.prev.set(None);
waiter.next.set(head);
waiter.tail.set(None);
}
// Release barrier synchronizes with Acquire barrier by threads doing Waiter::get_and_link_queue()
// to ensure that those threads see the waiter writes we did above when observing the state.
if let Err(e) = self.state.compare_exchange_weak(
*state,
new_state,
Ordering::Release,
Ordering::Relaxed,
) {
*state = e;
return false;
}
if (*state).address() & (QUEUED | QUEUE_LOCKED) == QUEUED {
self.link_queue_or_unpark(new_state);
}
true
}
#[cold]
unsafe fn link_queue_or_unpark(&self, mut state: *mut Waiter) {
loop {
assert_ne!(state.address() & QUEUED, 0);
assert_ne!(state.address() & QUEUE_LOCKED, 0);
// If the lock holders released the lock,
// we are now in charge of waking up threads since we hold the QUEUE_LOCKED bit.
// This is due to the lock-releasing thread skipping thread-wakeup
// if the QUEUE_LOCKED bit is set as we can take over its job.
if state.address() & LOCKED == 0 {
return self.unpark(state);
}
// Fix the prev links in the waiter queue now that we hold the QUEUE_LOCKED bit.
// Acquire barrier to ensure writes to waiters pushed to the queue happen before we start fixing it.
fence_acquire(&self.state);
let _ = Waiter::get_and_link_queue(state, |_| {});
// Finally, try to the release the QUEUE_LOCKED bit.
// Release barrier to ensure the writes we did above happen before the next QUEUE_LOCKED bit holder.
match self.state.compare_exchange_weak(
state,
state.map_address(|addr| addr & !QUEUE_LOCKED),
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => return,
Err(e) => state = e,
}
}
}
#[cold]
unsafe fn unpark(&self, mut state: *mut Waiter) {
loop {
assert_ne!(state.address() & QUEUED, 0);
assert_ne!(state.address() & QUEUE_LOCKED, 0);
// If the RwLock is locked by another thread while we're trying to wake one up,
// then bail by releasing the QUEUE_LOCKED bit as the lock holder can do the wake up instead.
// Release barrier to ensure the queue writes we've possibly done so far in Waiter::get_and_link_queue()
// below happen before the next QUEUE_LOCKED bit holder.
if state.address() & LOCKED != 0 {
match self.state.compare_exchange_weak(
state,
state.map_address(|addr| addr & !QUEUE_LOCKED),
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => return,
Err(e) => state = e,
}
continue;
}
// Fix and get the ends of the wait queue in order to wake the tail up.
// Acquire barrier ensures that writes to waiters pushed to the queue
// happen before we start fixing/getting it.
fence_acquire(&self.state);
let (head, tail) = Waiter::get_and_link_queue(state, |_| {});
// If the tail (the waiter to wake up) is a writer,
// then we can just wake up that one and leave the rest queued.
let is_writer = tail.as_ref().flags.get() != 0;
if is_writer {
// We only leave the reset queued if there is a "rest" to begin with.
if let Some(new_tail) = tail.as_ref().prev.get() {
// The tail is dequeued by updating the cached head references to it with the new tail.
// Unset the QUEUE_LOCKED bit now that we have dequeued the tail for waking.
// Release barrier ensures the head/tail updates happen before the next QUEUE_LOCKED bit owner.
head.as_ref().tail.set(Some(new_tail));
self.state
.fetch_sub(state.with_address(QUEUE_LOCKED), Ordering::Release);
// unpark_waiters() follows the queue backwards from the tail to the head using the `prev` field.
// Since we queue to the head, we dequeue from the tail.
// Given we're only taking up the tail, zero out its `prev` field.
tail.as_ref().prev.set(None);
return self.unpark_waiters(tail);
}
}
// The tail of the wait queue is a reader (not a writer).
//
// parking_lot would normally scan backwards from the tail to find all readers until the first writer for wakeup.
// The queue in most cases is generally small, so we can afford just waking everyone up instead
// with the assumption that they're all readers.
//
// To do that, we must zero out the queue portion of the state while also releasing the QUEUE_LOCKED bit.
// Release barrier ensures the head/tail access above happen before we release the QUEUE_LOCKED bit before wake up.
match self.state.compare_exchange_weak(
state,
state.map_address(|addr| addr & !(Waiter::MASK | QUEUED | QUEUE_LOCKED)),
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => return self.unpark_waiters(tail),
Err(e) => state = e,
}
}
}
#[cold]
unsafe fn unpark_waiters(&self, mut tail: NonNull<Waiter>) {
loop {
let waiting_on = tail.as_ref().waiting_on.get();
let waiting_on = waiting_on.expect("waking a waiter thats not waiting on anything");
assert_eq!(
waiting_on,
NonNull::from(self).cast(),
"waking a waiter thats not waiting on this lock",
);
let prev = tail.as_ref().prev.get();
tail.as_ref().parker.unpark();
tail = match prev {
Some(prev) => prev,
None => break,
};
}
}
}
/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// This lock uses a task-fair locking policy which avoids both reader and
/// writer starvation. This means that readers trying to acquire the lock will
/// block even if the lock is unlocked when there are writers waiting to acquire
/// the lock. Because of this, attempts to recursively acquire a read lock
/// within a single thread may result in a deadlock.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies `Send` to be shared across threads and `Sync` to
/// allow concurrent access through readers. The RAII guards returned from the
/// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
/// to allow access to the contained of the lock.
///
/// # Fairness
///
/// A typical unfair lock can often end up in a situation where a single thread
/// quickly acquires and releases the same lock in succession, which can starve
/// other threads waiting to acquire the rwlock. While this improves throughput
/// because it doesn't force a context switch when a thread tries to re-acquire
/// a rwlock it has just released, this can starve other threads.
///
/// This rwlock is unfair by default. This means that a thread which unlocks the
/// rwlock is allowed to re-acquire it again even when other threads are waiting
/// for the lock.
///
/// This greatly improves throughput (read "performance") but could potentially
/// starve an unlucky thread when there's constant lock contention. The rwlock
/// tries to at least wake up threads in the order that they we're queued as an
/// attempt to avoid starvation, but it is entirely up to the OS scheduler.
///
/// # Differences from the standard library `RwLock`
///
/// - Task-fair locking policy instead of an unspecified platform default.
/// - No poisoning, the lock is released normally on panic.
/// - Only requires 1 word of space, whereas the standard library boxes the
/// `RwLock` due to platform limitations.
/// - Can be statically constructed.
/// - Does not require any drop glue when dropped.
/// - Inline fast path for the uncontended case.
/// - Efficient handling of micro-contention using adaptive spinning.
/// - Allows raw locking & unlocking without a guard.
///
/// # Examples
///
/// ```
/// use usync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
/// let r1 = lock.read();
/// let r2 = lock.read();
/// assert_eq!(*r1, 5);
/// assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
/// let mut w = lock.write();
/// *w += 1;
/// assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
pub type RwLock<T> = lock_api::RwLock<RawRwLock, T>;
/// RAII structure used to release the shared read access of a lock when
/// dropped.
pub type RwLockReadGuard<'a, T> = lock_api::RwLockReadGuard<'a, RawRwLock, T>;
/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
pub type RwLockWriteGuard<'a, T> = lock_api::RwLockWriteGuard<'a, RawRwLock, T>;
/// An RAII read lock guard returned by `RwLockReadGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockReadGuard` and `RwLockReadGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockReadGuard<'a, T> = lock_api::MappedRwLockReadGuard<'a, RawRwLock, T>;
/// An RAII write lock guard returned by `RwLockWriteGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedRwLockWriteGuard` and `RwLockWriteGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedRwLockWriteGuard<'a, T> = lock_api::MappedRwLockWriteGuard<'a, RawRwLock, T>;
/// Creates a new instance of an `RwLock<T>` which is unlocked.
///
/// This allows creating a `RwLock<T>` in a constant context on stable Rust.
pub const fn const_rwlock<T>(value: T) -> RwLock<T> {
RwLock::const_new(<RawRwLock as lock_api::RawRwLock>::INIT, value)
}
#[cfg(test)]
mod tests {
use crate::RwLock;
use rand::Rng;
use std::{
sync::{
atomic::{AtomicUsize, Ordering},
mpsc::channel,
Arc,
},
thread,
};
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let l = RwLock::new(());
drop(l.read());
drop(l.write());
drop((l.read(), l.read()));
drop(l.write());
}
#[test]
fn frob() {
const N: u32 = 10;
const M: u32 = if cfg!(miri) { 100 } else { 1000 };
let r = Arc::new(RwLock::new(()));
let (tx, rx) = channel::<()>();
for _ in 0..N {
let tx = tx.clone();
let r = r.clone();
thread::spawn(move || {
let mut rng = rand::thread_rng();
for _ in 0..M {
if rng.gen_bool(1.0 / N as f64) {
drop(r.write());
} else {
drop(r.read());
}
}
drop(tx);
});
}
drop(tx);
let _ = rx.recv();
}
#[test]
fn test_rw_arc_no_poison_wr() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.write();
panic!();
})
.join();
let lock = arc.read();
assert_eq!(*lock, 1);
}
#[test]
fn test_rw_arc_no_poison_ww() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.write();
panic!();
})
.join();
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 _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.read();
panic!();
})
.join();
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 _: Result<(), _> = thread::spawn(move || {
let _lock = arc2.read();
panic!()
})
.join();
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();
thread::spawn(move || {
let mut lock = arc2.write();
for _ in 0..10 {
let tmp = *lock;
*lock = -1;
thread::yield_now();
*lock = tmp + 1;
}
tx.send(()).unwrap();
});
// Readers try to catch the writer in the act
let mut children = Vec::new();
for _ in 0..5 {
let arc3 = arc.clone();
children.push(thread::spawn(move || {
let lock = arc3.read();
assert!(*lock >= 0);
}));
}
// Wait for children to pass their asserts
for r in children {
assert!(r.join().is_ok());
}
// Wait for writer to finish
rx.recv().unwrap();
let lock = arc.read();
assert_eq!(*lock, 10);
}
#[test]
fn test_rw_arc_access_in_unwind() {
let arc = Arc::new(RwLock::new(1));
let arc2 = arc.clone();
let _ = thread::spawn(move || {
struct Unwinder {
i: Arc<RwLock<isize>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
let mut lock = self.i.write();
*lock += 1;
}
}
let _u = Unwinder { i: arc2 };
panic!();
})
.join();
let lock = arc.read();
assert_eq!(*lock, 2);
}
#[test]
fn test_rwlock_unsized() {
let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
{
let b = &mut *rw.write();
b[0] = 4;
b[2] = 5;
}
let comp: &[i32] = &[4, 2, 5];
assert_eq!(&*rw.read(), comp);
}
#[test]
fn test_rwlock_try_read() {
let lock = RwLock::new(0isize);
{
let read_guard = lock.read();
let read_result = lock.try_read();
assert!(
read_result.is_some(),
"try_read should succeed while read_guard is in scope"
);
drop(read_guard);
}
{
let write_guard = lock.write();
let read_result = lock.try_read();
assert!(
read_result.is_none(),
"try_read should fail while write_guard is in scope"
);
drop(write_guard);
}
}
#[test]
fn test_rwlock_try_write() {
let lock = RwLock::new(0isize);
{
let read_guard = lock.read();
let write_result = lock.try_write();
assert!(
write_result.is_none(),
"try_write should fail while read_guard is in scope"
);
assert!(lock.is_locked());
assert!(!lock.is_locked_exclusive());
drop(read_guard);
}
{
let write_guard = lock.write();
let write_result = lock.try_write();
assert!(
write_result.is_none(),
"try_write should fail while write_guard is in scope"
);
assert!(lock.is_locked());
assert!(lock.is_locked_exclusive());
drop(write_guard);
}
}
#[test]
fn test_into_inner() {
let m = RwLock::new(NonCopy(10));
assert_eq!(m.into_inner(), NonCopy(10));
}
#[test]
fn test_into_inner_drop() {
struct Foo(Arc<AtomicUsize>);
impl Drop for Foo {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::SeqCst);
}
}
let num_drops = Arc::new(AtomicUsize::new(0));
let m = RwLock::new(Foo(num_drops.clone()));
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
{
let _inner = m.into_inner();
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
}
assert_eq!(num_drops.load(Ordering::SeqCst), 1);
}
#[test]
fn test_get_mut() {
let mut m = RwLock::new(NonCopy(10));
*m.get_mut() = NonCopy(20);
assert_eq!(m.into_inner(), NonCopy(20));
}
#[test]
fn test_rwlockguard_sync() {
fn sync<T: Sync>(_: T) {}
let rwlock = RwLock::new(());
sync(rwlock.read());
sync(rwlock.write());
}
#[test]
fn test_rwlock_debug() {
let x = RwLock::new(vec![0u8, 10]);
assert_eq!(format!("{:?}", x), "RwLock { data: [0, 10] }");
let _lock = x.write();
assert_eq!(format!("{:?}", x), "RwLock { data: <locked> }");
}
#[test]
fn test_clone() {
let rwlock = RwLock::new(Arc::new(1));
let a = rwlock.read();
let b = a.clone();
assert_eq!(Arc::strong_count(&b), 2);
}
#[test]
fn test_parking_lot_issue_203() {
struct Bar(RwLock<()>);
impl Drop for Bar {
fn drop(&mut self) {
let _n = self.0.write();
}
}
thread_local! {
static B: Bar = Bar(RwLock::new(()));
}
thread::spawn(|| {
B.with(|_| ());
let a = RwLock::new(());
let _a = a.read();
})
.join()
.unwrap();
}
#[test]
fn test_rw_write_is_locked() {
let lock = RwLock::new(0isize);
{
let _read_guard = lock.read();
assert!(lock.is_locked());
assert!(!lock.is_locked_exclusive());
}
{
let _write_guard = lock.write();
assert!(lock.is_locked());
assert!(lock.is_locked_exclusive());
}
}
}