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Windows: Implement mutex using futex
Well, the Windows equivalent: `WaitOnAddress`, `WakeByAddressSingle` and `WakeByAddressAll`.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -1,96 +1,9 @@ | ||
| use crate::sync::atomic::{ | ||
| AtomicU32, | ||
| Ordering::{Acquire, Relaxed, Release}, | ||
| }; | ||
| use crate::sys::futex::{futex_wait, futex_wake}; | ||
|
|
||
| pub struct Mutex { | ||
| /// 0: unlocked | ||
| /// 1: locked, no other threads waiting | ||
| /// 2: locked, and other threads waiting (contended) | ||
| futex: AtomicU32, | ||
| } | ||
|
|
||
| impl Mutex { | ||
| #[inline] | ||
| pub const fn new() -> Self { | ||
| Self { futex: AtomicU32::new(0) } | ||
| } | ||
|
|
||
| #[inline] | ||
| pub fn try_lock(&self) -> bool { | ||
| self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok() | ||
| } | ||
|
|
||
| #[inline] | ||
| pub fn lock(&self) { | ||
| if self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_err() { | ||
| self.lock_contended(); | ||
| } | ||
| } | ||
|
|
||
| #[cold] | ||
| fn lock_contended(&self) { | ||
| // Spin first to speed things up if the lock is released quickly. | ||
| let mut state = self.spin(); | ||
|
|
||
| // If it's unlocked now, attempt to take the lock | ||
| // without marking it as contended. | ||
| if state == 0 { | ||
| match self.futex.compare_exchange(0, 1, Acquire, Relaxed) { | ||
| Ok(_) => return, // Locked! | ||
| Err(s) => state = s, | ||
| } | ||
| } | ||
|
|
||
| loop { | ||
| // Put the lock in contended state. | ||
| // We avoid an unnecessary write if it as already set to 2, | ||
| // to be friendlier for the caches. | ||
| if state != 2 && self.futex.swap(2, Acquire) == 0 { | ||
| // We changed it from 0 to 2, so we just successfully locked it. | ||
| return; | ||
| } | ||
|
|
||
| // Wait for the futex to change state, assuming it is still 2. | ||
| futex_wait(&self.futex, 2, None); | ||
|
|
||
| // Spin again after waking up. | ||
| state = self.spin(); | ||
| } | ||
| } | ||
|
|
||
| fn spin(&self) -> u32 { | ||
| let mut spin = 100; | ||
| loop { | ||
| // We only use `load` (and not `swap` or `compare_exchange`) | ||
| // while spinning, to be easier on the caches. | ||
| let state = self.futex.load(Relaxed); | ||
|
|
||
| // We stop spinning when the mutex is unlocked (0), | ||
| // but also when it's contended (2). | ||
| if state != 1 || spin == 0 { | ||
| return state; | ||
| } | ||
|
|
||
| crate::hint::spin_loop(); | ||
| spin -= 1; | ||
| } | ||
| } | ||
|
|
||
| #[inline] | ||
| pub unsafe fn unlock(&self) { | ||
| if self.futex.swap(0, Release) == 2 { | ||
| // We only wake up one thread. When that thread locks the mutex, it | ||
| // will mark the mutex as contended (2) (see lock_contended above), | ||
| // which makes sure that any other waiting threads will also be | ||
| // woken up eventually. | ||
| self.wake(); | ||
| } | ||
| } | ||
|
|
||
| #[cold] | ||
| fn wake(&self) { | ||
| futex_wake(&self.futex); | ||
| cfg_if::cfg_if! { | ||
| if #[cfg(windows)] { | ||
| mod windows; | ||
| pub use windows::*; | ||
| } else { | ||
| mod unix; | ||
| pub use unix::*; | ||
| } | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1 @@ | ||
|
|
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,65 @@ | ||
| use crate::sync::atomic::{ | ||
| AtomicI8, | ||
| Ordering::{Acquire, Relaxed, Release}, | ||
| }; | ||
| use crate::sys::api; | ||
|
|
||
| pub struct Mutex { | ||
| state: AtomicI8, | ||
| } | ||
|
|
||
| const UNLOCKED: i8 = 0; | ||
| const LOCKED: i8 = 1; | ||
| const CONTENDED: i8 = 2; | ||
|
|
||
| impl Mutex { | ||
| #[inline] | ||
| pub const fn new() -> Mutex { | ||
| Mutex { state: AtomicI8::new(UNLOCKED) } | ||
| } | ||
|
|
||
| #[inline] | ||
| pub fn lock(&self) { | ||
| if let Err(state) = self.state.compare_exchange(UNLOCKED, LOCKED, Acquire, Relaxed) { | ||
| self.lock_contended(state) | ||
| } | ||
| } | ||
|
|
||
| #[cold] | ||
| fn lock_contended(&self, mut state: i8) { | ||
| // Note: WaitOnAddress is already quite spin-happy so we don't do any further spinning on top. | ||
| loop { | ||
| // Put the lock in contended state. | ||
| // We avoid an unnecessary write if it as already set to CONTENDED, | ||
| // to be friendlier for the caches. | ||
| if state != CONTENDED && self.state.swap(CONTENDED, Acquire) == UNLOCKED { | ||
| // We changed it from UNLOCKED to CONTENDED, so we just successfully locked it. | ||
| return; | ||
| } | ||
| // Wait for the futex to change state, assuming it is still CONTENDED. | ||
| api::wait_on_address(&self.state, CONTENDED, None); | ||
| state = self.state.load(Relaxed); | ||
| } | ||
| } | ||
|
|
||
| #[inline] | ||
| pub fn try_lock(&self) -> bool { | ||
| self.state.compare_exchange(UNLOCKED, LOCKED, Acquire, Relaxed).is_ok() | ||
| } | ||
|
|
||
| #[inline] | ||
| pub unsafe fn unlock(&self) { | ||
| if self.state.swap(UNLOCKED, Release) == CONTENDED { | ||
| // We only wake up one thread. When that thread locks the mutex, it | ||
| // will mark the mutex as CONTENDED (see lock_contended above), | ||
| // which makes sure that any other waiting threads will also be | ||
| // woken up eventually. | ||
| self.wake(); | ||
| } | ||
| } | ||
|
|
||
| #[cold] | ||
| fn wake(&self) { | ||
| api::wake_by_address_single(&self.state); | ||
| } | ||
| } |
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