Windows: Implement mutex using futex

Well, the Windows equivalent: `WaitOnAddress`, `WakeByAddressSingle` and `WakeByAddressAll`.

7 files changed, 45 insertions, 30 deletions

diff --git a/library/std/src/sys/locks/condvar/mod.rs b/library/std/src/sys/locks/condvar/mod.rs
index 126a42a2a4c..6849cacf88e 100644
--- a/library/std/src/sys/locks/condvar/mod.rs
+++ b/library/std/src/sys/locks/condvar/mod.rs
@@ -1,5 +1,6 @@
 cfg_if::cfg_if! {
     if #[cfg(any(
+        all(target_os = "windows", not(target_vendor="win7")),
         target_os = "linux",
         target_os = "android",
         target_os = "freebsd",
@@ -14,9 +15,9 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_family = "unix")] {
         mod pthread;
         pub use pthread::Condvar;
-    } else if #[cfg(target_os = "windows")] {
-        mod windows;
-        pub use windows::Condvar;
+    } else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+        mod windows7;
+        pub use windows7::Condvar;
     } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
         mod sgx;
         pub use sgx::Condvar;
diff --git a/library/std/src/sys/locks/condvar/windows.rs b/library/std/src/sys/locks/condvar/windows7.rs
index 28a288335d2..28a288335d2 100644
--- a/library/std/src/sys/locks/condvar/windows.rs
+++ b/library/std/src/sys/locks/condvar/windows7.rs
diff --git a/library/std/src/sys/locks/mutex/futex.rs b/library/std/src/sys/locks/mutex/futex.rs
index c01229586c3..7427cae94d6 100644
--- a/library/std/src/sys/locks/mutex/futex.rs
+++ b/library/std/src/sys/locks/mutex/futex.rs
@@ -1,30 +1,42 @@
 use crate::sync::atomic::{
-    AtomicU32,
+    self,
     Ordering::{Acquire, Relaxed, Release},
 };
 use crate::sys::futex::{futex_wait, futex_wake};
 
+cfg_if::cfg_if! {
+if #[cfg(windows)] {
+    // On Windows we can have a smol futex
+    type Atomic = atomic::AtomicU8;
+    type State = u8;
+} else {
+    type Atomic = atomic::AtomicU32;
+    type State = u32;
+}
+}
+
 pub struct Mutex {
-    /// 0: unlocked
-    /// 1: locked, no other threads waiting
-    /// 2: locked, and other threads waiting (contended)
-    futex: AtomicU32,
+    futex: Atomic,
 }
 
+const UNLOCKED: State = 0;
+const LOCKED: State = 1; // locked, no other threads waiting
+const CONTENDED: State = 2; // locked, and other threads waiting (contended)
+
 impl Mutex {
     #[inline]
     pub const fn new() -> Self {
-        Self { futex: AtomicU32::new(0) }
+        Self { futex: Atomic::new(UNLOCKED) }
     }
 
     #[inline]
     pub fn try_lock(&self) -> bool {
-        self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_ok()
+        self.futex.compare_exchange(UNLOCKED, LOCKED, Acquire, Relaxed).is_ok()
     }
 
     #[inline]
     pub fn lock(&self) {
-        if self.futex.compare_exchange(0, 1, Acquire, Relaxed).is_err() {
+        if self.futex.compare_exchange(UNLOCKED, LOCKED, Acquire, Relaxed).is_err() {
             self.lock_contended();
         }
     }
@@ -36,8 +48,8 @@ impl Mutex {
 
         // 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) {
+        if state == UNLOCKED {
+            match self.futex.compare_exchange(UNLOCKED, LOCKED, Acquire, Relaxed) {
                 Ok(_) => return, // Locked!
                 Err(s) => state = s,
             }
@@ -45,31 +57,31 @@ impl Mutex {
 
         loop {
             // Put the lock in contended state.
-            // We avoid an unnecessary write if it as already set to 2,
+            // We avoid an unnecessary write if it as already set to CONTENDED,
             // 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.
+            if state != CONTENDED && self.futex.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 2.
-            futex_wait(&self.futex, 2, None);
+            // Wait for the futex to change state, assuming it is still CONTENDED.
+            futex_wait(&self.futex, CONTENDED, None);
 
             // Spin again after waking up.
             state = self.spin();
         }
     }
 
-    fn spin(&self) -> u32 {
+    fn spin(&self) -> State {
         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 {
+            // We stop spinning when the mutex is UNLOCKED,
+            // but also when it's CONTENDED.
+            if state != LOCKED || spin == 0 {
                 return state;
             }
 
@@ -80,9 +92,9 @@ impl Mutex {
 
     #[inline]
     pub unsafe fn unlock(&self) {
-        if self.futex.swap(0, Release) == 2 {
+        if self.futex.swap(UNLOCKED, Release) == CONTENDED {
             // We only wake up one thread. When that thread locks the mutex, it
-            // will mark the mutex as contended (2) (see lock_contended above),
+            // 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();
diff --git a/library/std/src/sys/locks/mutex/mod.rs b/library/std/src/sys/locks/mutex/mod.rs
index 710cb91fb14..73d9bd273de 100644
--- a/library/std/src/sys/locks/mutex/mod.rs
+++ b/library/std/src/sys/locks/mutex/mod.rs
@@ -1,5 +1,6 @@
 cfg_if::cfg_if! {
     if #[cfg(any(
+        all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
         target_os = "freebsd",
@@ -19,9 +20,9 @@ cfg_if::cfg_if! {
     ))] {
         mod pthread;
         pub use pthread::{Mutex, raw};
-    } else if #[cfg(target_os = "windows")] {
-        mod windows;
-        pub use windows::{Mutex, raw};
+    } else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+        mod windows7;
+        pub use windows7::{Mutex, raw};
     } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
         mod sgx;
         pub use sgx::Mutex;
diff --git a/library/std/src/sys/locks/mutex/windows.rs b/library/std/src/sys/locks/mutex/windows7.rs
index ef2f84082cd..ef2f84082cd 100644
--- a/library/std/src/sys/locks/mutex/windows.rs
+++ b/library/std/src/sys/locks/mutex/windows7.rs
diff --git a/library/std/src/sys/locks/rwlock/mod.rs b/library/std/src/sys/locks/rwlock/mod.rs
index 0564f1fe6fa..675931c64bd 100644
--- a/library/std/src/sys/locks/rwlock/mod.rs
+++ b/library/std/src/sys/locks/rwlock/mod.rs
@@ -1,5 +1,6 @@
 cfg_if::cfg_if! {
     if #[cfg(any(
+        all(target_os = "windows", not(target_vendor = "win7")),
         target_os = "linux",
         target_os = "android",
         target_os = "freebsd",
@@ -14,9 +15,9 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_family = "unix")] {
         mod queue;
         pub use queue::RwLock;
-    } else if #[cfg(target_os = "windows")] {
-        mod windows;
-        pub use windows::RwLock;
+    } else if #[cfg(all(target_os = "windows", target_vendor = "win7"))] {
+        mod windows7;
+        pub use windows7::RwLock;
     } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
         mod sgx;
         pub use sgx::RwLock;
diff --git a/library/std/src/sys/locks/rwlock/windows.rs b/library/std/src/sys/locks/rwlock/windows7.rs
index e69415baac4..e69415baac4 100644
--- a/library/std/src/sys/locks/rwlock/windows.rs
+++ b/library/std/src/sys/locks/rwlock/windows7.rs