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inline some single-use functions, add documentation

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joboet 2023-04-22 15:58:32 +02:00
parent 16aae04f68
commit 1fd9f7898e
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1 changed files with 23 additions and 23 deletions
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46
library/std/src/sys/pal/unix/locks/queue_rwlock.rs
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@ -199,20 +199,6 @@ impl Node {
}
}
/// Set the `next` field depending on the lock state. If there are threads
/// queued, the `next` field will be set to a pointer to the next node in
/// the queue. Otherwise the `next` field will be set to the lock count if
/// the state is read-locked or to zero if it is write-locked.
fn set_state(&mut self, state: State) {
self.next.0 = AtomicPtr::new(state.mask(MASK).cast());
}
/// Assuming the node contains a reader lock count, decrement that count.
/// Returns `true` if this thread was the last lock owner.
fn decrement_count(&self) -> bool {
self.next.0.fetch_byte_sub(SINGLE, AcqRel).addr() - SINGLE == 0
}
/// Prepare this node for waiting.
fn prepare(&mut self) {
// Fall back to creating an unnamed `Thread` handle to allow locking in
@ -312,10 +298,11 @@ impl RwLock {
#[inline]
pub fn try_write(&self) -> bool {
// This is lowered to a single atomic instruction on most modern processors
// (e.g. "lock bts" on x86 and "ldseta" on modern AArch64), and therefore
// is more efficient than `fetch_update(lock(true))`, which can spuriously
// fail if a new node is appended to the queue.
// Atomically set the `LOCKED` bit. This is lowered to a single atomic
// instruction on most modern processors (e.g. "lock bts" on x86 and
// "ldseta" on modern AArch64), and therefore is more efficient than
// `fetch_update(lock(true))`, which can spuriously fail if a new node
// is appended to the queue.
self.state.fetch_or(LOCKED, Acquire).addr() & LOCKED == 0
}
@ -351,7 +338,12 @@ impl RwLock {
} else {
// Fall back to parking. First, prepare the node.
node.prepare();
node.set_state(state);
// If there are threads queued, set the `next` field to a
// pointer to the next node in the queue. Otherwise set it to
// the lock count if the state is read-locked or to zero if it
// is write-locked.
node.next.0 = AtomicPtr::new(state.mask(MASK).cast());
node.prev = AtomicLink::new(None);
let mut next = ptr::from_ref(&node)
.map_addr(|addr| addr | QUEUED | (state.addr() & LOCKED))
@ -370,8 +362,8 @@ impl RwLock {
next = next.map_addr(|addr| addr | QUEUE_LOCKED);
}
// Use release ordering to propagate our changes to the waking
// thread.
// Register the node, using release ordering to propagate our
// changes to the waking thread.
if let Err(new) = self.state.compare_exchange_weak(state, next, AcqRel, Relaxed) {
// The state has changed, just try again.
state = new;
@ -430,8 +422,16 @@ impl RwLock {
// The state was observed with acquire ordering above, so the current
// thread will observe all node initializations.
let tail = unsafe { find_tail(to_node(state)) };
let was_last = unsafe { tail.as_ref().decrement_count() };
// SAFETY:
// Because new read-locks cannot be acquired while threads are queued,
// all queue-lock owners will observe the set `LOCKED` bit. Because they
// do not modify the queue while there is a lock owner, the queue will
// not be removed from here.
let tail = unsafe { find_tail(to_node(state)).as_ref() };
// The lock count is stored in the `next` field of `tail`.
// Decrement it, making sure to observe all changes made to the queue
// by the other lock owners by using acquire-release ordering.
let was_last = tail.next.0.fetch_byte_sub(SINGLE, AcqRel).addr() - SINGLE == 0;
if was_last {
// SAFETY:
// Other threads cannot read-lock while threads are queued. Also,