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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:11:38 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:12:43 +0000 |
commit | cf94bdc0742c13e2a0cac864c478b8626b266e1b (patch) | |
tree | 044670aa50cc5e2b4229aa0b6b3df6676730c0a6 /library/std/src/sys_common/once/futex.rs | |
parent | Adding debian version 1.65.0+dfsg1-2. (diff) | |
download | rustc-cf94bdc0742c13e2a0cac864c478b8626b266e1b.tar.xz rustc-cf94bdc0742c13e2a0cac864c478b8626b266e1b.zip |
Merging upstream version 1.66.0+dfsg1.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | library/std/src/sys_common/once/futex.rs | 134 |
1 files changed, 134 insertions, 0 deletions
diff --git a/library/std/src/sys_common/once/futex.rs b/library/std/src/sys_common/once/futex.rs new file mode 100644 index 000000000..5c7e6c013 --- /dev/null +++ b/library/std/src/sys_common/once/futex.rs @@ -0,0 +1,134 @@ +use crate::cell::Cell; +use crate::sync as public; +use crate::sync::atomic::{ + AtomicU32, + Ordering::{Acquire, Relaxed, Release}, +}; +use crate::sys::futex::{futex_wait, futex_wake_all}; + +// On some platforms, the OS is very nice and handles the waiter queue for us. +// This means we only need one atomic value with 5 states: + +/// No initialization has run yet, and no thread is currently using the Once. +const INCOMPLETE: u32 = 0; +/// 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. +const POISONED: u32 = 1; +/// Some thread is currently attempting to run initialization. It may succeed, +/// so all future threads need to wait for it to finish. +const RUNNING: u32 = 2; +/// Some thread is currently attempting to run initialization and there are threads +/// waiting for it to finish. +const QUEUED: u32 = 3; +/// Initialization has completed and all future calls should finish immediately. +const COMPLETE: u32 = 4; + +// Threads wait by setting the state to QUEUED and calling `futex_wait` on the state +// variable. When the running thread finishes, it will wake all waiting threads using +// `futex_wake_all`. + +pub struct OnceState { + poisoned: bool, + set_state_to: Cell<u32>, +} + +impl OnceState { + #[inline] + pub fn is_poisoned(&self) -> bool { + self.poisoned + } + + #[inline] + pub fn poison(&self) { + self.set_state_to.set(POISONED); + } +} + +struct CompletionGuard<'a> { + state: &'a AtomicU32, + set_state_on_drop_to: u32, +} + +impl<'a> Drop for CompletionGuard<'a> { + fn drop(&mut self) { + // Use release ordering to propagate changes to all threads checking + // up on the Once. `futex_wake_all` does its own synchronization, hence + // we do not need `AcqRel`. + if self.state.swap(self.set_state_on_drop_to, Release) == QUEUED { + futex_wake_all(&self.state); + } + } +} + +pub struct Once { + state: AtomicU32, +} + +impl Once { + #[inline] + pub const fn new() -> Once { + Once { state: AtomicU32::new(INCOMPLETE) } + } + + #[inline] + pub fn is_completed(&self) -> bool { + // Use acquire ordering to make all initialization changes visible to the + // current thread. + self.state.load(Acquire) == COMPLETE + } + + // This uses FnMut to match the API of the generic implementation. As this + // implementation is quite light-weight, it is generic over the closure and + // so avoids the cost of dynamic dispatch. + #[cold] + #[track_caller] + pub fn call(&self, ignore_poisoning: bool, f: &mut impl FnMut(&public::OnceState)) { + let mut state = self.state.load(Acquire); + loop { + match state { + POISONED if !ignore_poisoning => { + // Panic to propagate the poison. + panic!("Once instance has previously been poisoned"); + } + INCOMPLETE | POISONED => { + // Try to register the current thread as the one running. + if let Err(new) = + self.state.compare_exchange_weak(state, RUNNING, Acquire, Acquire) + { + state = new; + continue; + } + // `waiter_queue` will manage other waiting threads, and + // wake them up on drop. + let mut waiter_queue = + CompletionGuard { state: &self.state, set_state_on_drop_to: POISONED }; + // Run the function, letting it know if we're poisoned or not. + let f_state = public::OnceState { + inner: OnceState { + poisoned: state == POISONED, + set_state_to: Cell::new(COMPLETE), + }, + }; + f(&f_state); + waiter_queue.set_state_on_drop_to = f_state.inner.set_state_to.get(); + return; + } + RUNNING | QUEUED => { + // Set the state to QUEUED if it is not already. + if state == RUNNING + && let Err(new) = self.state.compare_exchange_weak(RUNNING, QUEUED, Relaxed, Acquire) + { + state = new; + continue; + } + + futex_wait(&self.state, QUEUED, None); + state = self.state.load(Acquire); + } + COMPLETE => return, + _ => unreachable!("state is never set to invalid values"), + } + } + } +} |