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Diffstat (limited to 'library/std/src/sys_common/once/queue.rs')
| -rw-r--r-- | library/std/src/sys_common/once/queue.rs | 283 |
1 files changed, 283 insertions, 0 deletions
diff --git a/library/std/src/sys_common/once/queue.rs b/library/std/src/sys_common/once/queue.rs new file mode 100644 index 000000000..d953a6745 --- /dev/null +++ b/library/std/src/sys_common/once/queue.rs @@ -0,0 +1,283 @@ +// Each `Once` has one word of atomic state, and this state is CAS'd on to +// determine what to do. There are four possible state of a `Once`: +// +// * Incomplete - no initialization has run yet, and no thread is currently +// using the Once. +// * Poisoned - some thread has previously attempted to initialize the Once, but +// it panicked, so the Once is now poisoned. There are no other +// threads currently accessing this Once. +// * Running - some thread is currently attempting to run initialization. It may +// succeed, so all future threads need to wait for it to finish. +// Note that this state is accompanied with a payload, described +// below. +// * Complete - initialization has completed and all future calls should finish +// immediately. +// +// With 4 states we need 2 bits to encode this, and we use the remaining bits +// in the word we have allocated as a queue of threads waiting for the thread +// responsible for entering the RUNNING state. This queue is just a linked list +// of Waiter nodes which is monotonically increasing in size. Each node is +// allocated on the stack, and whenever the running closure finishes it will +// consume the entire queue and notify all waiters they should try again. +// +// You'll find a few more details in the implementation, but that's the gist of +// it! +// +// Atomic orderings: +// When running `Once` we deal with multiple atomics: +// `Once.state_and_queue` and an unknown number of `Waiter.signaled`. +// * `state_and_queue` is used (1) as a state flag, (2) for synchronizing the +// result of the `Once`, and (3) for synchronizing `Waiter` nodes. +// - At the end of the `call` function we have to make sure the result +// of the `Once` is acquired. So every load which can be the only one to +// load COMPLETED must have at least acquire ordering, which means all +// three of them. +// - `WaiterQueue::drop` is the only place that may store COMPLETED, and +// must do so with release ordering to make the result available. +// - `wait` inserts `Waiter` nodes as a pointer in `state_and_queue`, and +// needs to make the nodes available with release ordering. The load in +// its `compare_exchange` can be relaxed because it only has to compare +// the atomic, not to read other data. +// - `WaiterQueue::drop` must see the `Waiter` nodes, so it must load +// `state_and_queue` with acquire ordering. +// - There is just one store where `state_and_queue` is used only as a +// state flag, without having to synchronize data: switching the state +// from INCOMPLETE to RUNNING in `call`. This store can be Relaxed, +// but the read has to be Acquire because of the requirements mentioned +// above. +// * `Waiter.signaled` is both used as a flag, and to protect a field with +// interior mutability in `Waiter`. `Waiter.thread` is changed in +// `WaiterQueue::drop` which then sets `signaled` with release ordering. +// After `wait` loads `signaled` with acquire ordering and sees it is true, +// it needs to see the changes to drop the `Waiter` struct correctly. +// * There is one place where the two atomics `Once.state_and_queue` and +// `Waiter.signaled` come together, and might be reordered by the compiler or +// processor. Because both use acquire ordering such a reordering is not +// allowed, so no need for `SeqCst`. + +use crate::cell::Cell; +use crate::fmt; +use crate::ptr; +use crate::sync as public; +use crate::sync::atomic::{AtomicBool, AtomicPtr, Ordering}; +use crate::thread::{self, Thread}; + +type Masked = (); + +pub struct Once { + state_and_queue: AtomicPtr<Masked>, +} + +pub struct OnceState { + poisoned: bool, + set_state_on_drop_to: Cell<*mut Masked>, +} + +// Four states that a Once can be in, encoded into the lower bits of +// `state_and_queue` in the Once structure. +const INCOMPLETE: usize = 0x0; +const POISONED: usize = 0x1; +const RUNNING: usize = 0x2; +const COMPLETE: usize = 0x3; + +// Mask to learn about the state. All other bits are the queue of waiters if +// this is in the RUNNING state. +const STATE_MASK: usize = 0x3; + +// Representation of a node in the linked list of waiters, used while in the +// RUNNING state. +// Note: `Waiter` can't hold a mutable pointer to the next thread, because then +// `wait` would both hand out a mutable reference to its `Waiter` node, and keep +// a shared reference to check `signaled`. Instead we hold shared references and +// use interior mutability. +#[repr(align(4))] // Ensure the two lower bits are free to use as state bits. +struct Waiter { + thread: Cell<Option<Thread>>, + signaled: AtomicBool, + next: *const Waiter, +} + +// Head of a linked list of waiters. +// Every node is a struct on the stack of a waiting thread. +// Will wake up the waiters when it gets dropped, i.e. also on panic. +struct WaiterQueue<'a> { + state_and_queue: &'a AtomicPtr<Masked>, + set_state_on_drop_to: *mut Masked, +} + +impl Once { + #[inline] + #[rustc_const_stable(feature = "const_once_new", since = "1.32.0")] + pub const fn new() -> Once { + Once { state_and_queue: AtomicPtr::new(ptr::invalid_mut(INCOMPLETE)) } + } + + #[inline] + pub fn is_completed(&self) -> bool { + // An `Acquire` load is enough because that makes all the initialization + // operations visible to us, and, this being a fast path, weaker + // ordering helps with performance. This `Acquire` synchronizes with + // `Release` operations on the slow path. + self.state_and_queue.load(Ordering::Acquire).addr() == COMPLETE + } + + // This is a non-generic function to reduce the monomorphization cost of + // using `call_once` (this isn't exactly a trivial or small implementation). + // + // Additionally, this is tagged with `#[cold]` as it should indeed be cold + // and it helps let LLVM know that calls to this function should be off the + // fast path. Essentially, this should help generate more straight line code + // in LLVM. + // + // Finally, this takes an `FnMut` instead of a `FnOnce` because there's + // currently no way to take an `FnOnce` and call it via virtual dispatch + // without some allocation overhead. + #[cold] + #[track_caller] + pub fn call(&self, ignore_poisoning: bool, init: &mut dyn FnMut(&public::OnceState)) { + let mut state_and_queue = self.state_and_queue.load(Ordering::Acquire); + loop { + match state_and_queue.addr() { + COMPLETE => break, + POISONED if !ignore_poisoning => { + // Panic to propagate the poison. + panic!("Once instance has previously been poisoned"); + } + POISONED | INCOMPLETE => { + // Try to register this thread as the one RUNNING. + let exchange_result = self.state_and_queue.compare_exchange( + state_and_queue, + ptr::invalid_mut(RUNNING), + Ordering::Acquire, + Ordering::Acquire, + ); + if let Err(old) = exchange_result { + state_and_queue = old; + continue; + } + // `waiter_queue` will manage other waiting threads, and + // wake them up on drop. + let mut waiter_queue = WaiterQueue { + state_and_queue: &self.state_and_queue, + set_state_on_drop_to: ptr::invalid_mut(POISONED), + }; + // Run the initialization function, letting it know if we're + // poisoned or not. + let init_state = public::OnceState { + inner: OnceState { + poisoned: state_and_queue.addr() == POISONED, + set_state_on_drop_to: Cell::new(ptr::invalid_mut(COMPLETE)), + }, + }; + init(&init_state); + waiter_queue.set_state_on_drop_to = init_state.inner.set_state_on_drop_to.get(); + break; + } + _ => { + // All other values must be RUNNING with possibly a + // pointer to the waiter queue in the more significant bits. + assert!(state_and_queue.addr() & STATE_MASK == RUNNING); + wait(&self.state_and_queue, state_and_queue); + state_and_queue = self.state_and_queue.load(Ordering::Acquire); + } + } + } + } +} + +fn wait(state_and_queue: &AtomicPtr<Masked>, mut current_state: *mut Masked) { + // Note: the following code was carefully written to avoid creating a + // mutable reference to `node` that gets aliased. + loop { + // Don't queue this thread if the status is no longer running, + // otherwise we will not be woken up. + if current_state.addr() & STATE_MASK != RUNNING { + return; + } + + // Create the node for our current thread. + let node = Waiter { + thread: Cell::new(Some(thread::current())), + signaled: AtomicBool::new(false), + next: current_state.with_addr(current_state.addr() & !STATE_MASK) as *const Waiter, + }; + let me = &node as *const Waiter as *const Masked as *mut Masked; + + // Try to slide in the node at the head of the linked list, making sure + // that another thread didn't just replace the head of the linked list. + let exchange_result = state_and_queue.compare_exchange( + current_state, + me.with_addr(me.addr() | RUNNING), + Ordering::Release, + Ordering::Relaxed, + ); + if let Err(old) = exchange_result { + current_state = old; + continue; + } + + // We have enqueued ourselves, now lets wait. + // It is important not to return before being signaled, otherwise we + // would drop our `Waiter` node and leave a hole in the linked list + // (and a dangling reference). Guard against spurious wakeups by + // reparking ourselves until we are signaled. + while !node.signaled.load(Ordering::Acquire) { + // If the managing thread happens to signal and unpark us before we + // can park ourselves, the result could be this thread never gets + // unparked. Luckily `park` comes with the guarantee that if it got + // an `unpark` just before on an unparked thread it does not park. + thread::park(); + } + break; + } +} + +#[stable(feature = "std_debug", since = "1.16.0")] +impl fmt::Debug for Once { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.debug_struct("Once").finish_non_exhaustive() + } +} + +impl Drop for WaiterQueue<'_> { + fn drop(&mut self) { + // Swap out our state with however we finished. + let state_and_queue = + self.state_and_queue.swap(self.set_state_on_drop_to, Ordering::AcqRel); + + // We should only ever see an old state which was RUNNING. + assert_eq!(state_and_queue.addr() & STATE_MASK, RUNNING); + + // Walk the entire linked list of waiters and wake them up (in lifo + // order, last to register is first to wake up). + unsafe { + // Right after setting `node.signaled = true` the other thread may + // free `node` if there happens to be has a spurious wakeup. + // So we have to take out the `thread` field and copy the pointer to + // `next` first. + let mut queue = + state_and_queue.with_addr(state_and_queue.addr() & !STATE_MASK) as *const Waiter; + while !queue.is_null() { + let next = (*queue).next; + let thread = (*queue).thread.take().unwrap(); + (*queue).signaled.store(true, Ordering::Release); + // ^- FIXME (maybe): This is another case of issue #55005 + // `store()` has a potentially dangling ref to `signaled`. + queue = next; + thread.unpark(); + } + } + } +} + +impl OnceState { + #[inline] + pub fn is_poisoned(&self) -> bool { + self.poisoned + } + + #[inline] + pub fn poison(&self) { + self.set_state_on_drop_to.set(ptr::invalid_mut(POISONED)); + } +} |