Rollup merge of #96040 - m-ou-se:futex-u32, r=Amanieu

Use u32 instead of i32 for futexes.

This changes futexes from i32 to u32. The [Linux man page](https://man7.org/linux/man-pages/man2/futex.2.html) uses `uint32_t` for them, so I'm not sure why I used i32 for them. Maybe because I first used them for thread parkers, where I used -1, 0, and 1 as the states.

(Wasm's `memory.atomic.wait32` does use `i32`, because wasm doesn't support `u32`.)

It doesn't matter much, but using the unsigned type probably results in fewer surprises when shifting bits around or using comparison operators.

r? `@Amanieu`

library/std/src/sys/unix/futex.rs

@@ -4,7 +4,7 @@
     all(target_os = "emscripten", target_feature = "atomics")
 ))]
 
-use crate::sync::atomic::AtomicI32;
+use crate::sync::atomic::AtomicU32;
 use crate::time::Duration;
 
 /// Wait for a futex_wake operation to wake us.
@@ -13,7 +13,7 @@ use crate::time::Duration;
 ///
 /// Returns false on timeout, and true in all other cases.
 #[cfg(any(target_os = "linux", target_os = "android"))]
-pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) -> bool {
+pub fn futex_wait(futex: &AtomicU32, expected: u32, timeout: Option<Duration>) -> bool {
     use super::time::Timespec;
     use crate::ptr::null;
     use crate::sync::atomic::Ordering::Relaxed;
@@ -35,7 +35,7 @@ pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) -
         let r = unsafe {
             libc::syscall(
                 libc::SYS_futex,
-                futex as *const AtomicI32,
+                futex as *const AtomicU32,
                 libc::FUTEX_WAIT_BITSET | libc::FUTEX_PRIVATE_FLAG,
                 expected,
                 timespec.as_ref().map_or(null(), |t| &t.t as *const libc::timespec),
@@ -53,21 +53,19 @@ pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) -
 }
 
 #[cfg(target_os = "emscripten")]
-pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) {
+pub fn futex_wait(futex: &AtomicU32, expected: u32, timeout: Option<Duration>) {
     extern "C" {
         fn emscripten_futex_wait(
-            addr: *const AtomicI32,
+            addr: *const AtomicU32,
             val: libc::c_uint,
             max_wait_ms: libc::c_double,
         ) -> libc::c_int;
     }
 
     unsafe {
         emscripten_futex_wait(
-            futex as *const AtomicI32,
-            // `val` is declared unsigned to match the Emscripten headers, but since it's used as
-            // an opaque value, we can ignore the meaning of signed vs. unsigned and cast here.
-            expected as libc::c_uint,
+            futex,
+            expected,
             timeout.map_or(crate::f64::INFINITY, |d| d.as_secs_f64() * 1000.0),
         );
     }
@@ -78,11 +76,11 @@ pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) {
 /// Returns true if this actually woke up such a thread,
 /// or false if no thread was waiting on this futex.
 #[cfg(any(target_os = "linux", target_os = "android"))]
-pub fn futex_wake(futex: &AtomicI32) -> bool {
+pub fn futex_wake(futex: &AtomicU32) -> bool {
     unsafe {
         libc::syscall(
             libc::SYS_futex,
-            futex as *const AtomicI32,
+            futex as *const AtomicU32,
             libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG,
             1,
         ) > 0
@@ -91,22 +89,22 @@ pub fn futex_wake(futex: &AtomicI32) -> bool {
 
 /// Wake up all threads that are waiting on futex_wait on this futex.
 #[cfg(any(target_os = "linux", target_os = "android"))]
-pub fn futex_wake_all(futex: &AtomicI32) {
+pub fn futex_wake_all(futex: &AtomicU32) {
     unsafe {
         libc::syscall(
             libc::SYS_futex,
-            futex as *const AtomicI32,
+            futex as *const AtomicU32,
             libc::FUTEX_WAKE | libc::FUTEX_PRIVATE_FLAG,
             i32::MAX,
         );
     }
 }
 
 #[cfg(target_os = "emscripten")]
-pub fn futex_wake(futex: &AtomicI32) -> bool {
+pub fn futex_wake(futex: &AtomicU32) -> bool {
     extern "C" {
-        fn emscripten_futex_wake(addr: *const AtomicI32, count: libc::c_int) -> libc::c_int;
+        fn emscripten_futex_wake(addr: *const AtomicU32, count: libc::c_int) -> libc::c_int;
     }
 
-    unsafe { emscripten_futex_wake(futex as *const AtomicI32, 1) > 0 }
+    unsafe { emscripten_futex_wake(futex, 1) > 0 }
 }

library/std/src/sys/unix/locks/futex.rs

@@ -1,6 +1,6 @@
 use crate::cell::UnsafeCell;
 use crate::sync::atomic::{
-    AtomicI32, AtomicUsize,
+    AtomicU32, AtomicUsize,
     Ordering::{Acquire, Relaxed, Release},
 };
 use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
@@ -13,13 +13,13 @@ pub struct Mutex {
     /// 0: unlocked
     /// 1: locked, no other threads waiting
     /// 2: locked, and other threads waiting (contended)
-    futex: AtomicI32,
+    futex: AtomicU32,
 }
 
 impl Mutex {
     #[inline]
     pub const fn new() -> Self {
-        Self { futex: AtomicI32::new(0) }
+        Self { futex: AtomicU32::new(0) }
     }
 
     #[inline]
@@ -71,7 +71,7 @@ impl Mutex {
         }
     }
 
-    fn spin(&self) -> i32 {
+    fn spin(&self) -> u32 {
         let mut spin = 100;
         loop {
             // We only use `load` (and not `swap` or `compare_exchange`)
@@ -110,13 +110,13 @@ pub struct Condvar {
     // The value of this atomic is simply incremented on every notification.
     // This is used by `.wait()` to not miss any notifications after
     // unlocking the mutex and before waiting for notifications.
-    futex: AtomicI32,
+    futex: AtomicU32,
 }
 
 impl Condvar {
     #[inline]
     pub const fn new() -> Self {
-        Self { futex: AtomicI32::new(0) }
+        Self { futex: AtomicU32::new(0) }
     }
 
     #[inline]

library/std/src/sys/unix/locks/futex_rwlock.rs

@@ -1,5 +1,5 @@
 use crate::sync::atomic::{
-    AtomicI32,
+    AtomicU32,
     Ordering::{Acquire, Relaxed, Release},
 };
 use crate::sys::futex::{futex_wait, futex_wake, futex_wake_all};
@@ -14,36 +14,36 @@ pub struct RwLock {
     //   0x3FFF_FFFF: Write locked
     // Bit 30: Readers are waiting on this futex.
     // Bit 31: Writers are waiting on the writer_notify futex.
-    state: AtomicI32,
+    state: AtomicU32,
     // The 'condition variable' to notify writers through.
     // Incremented on every signal.
-    writer_notify: AtomicI32,
+    writer_notify: AtomicU32,
 }
 
-const READ_LOCKED: i32 = 1;
-const MASK: i32 = (1 << 30) - 1;
-const WRITE_LOCKED: i32 = MASK;
-const MAX_READERS: i32 = MASK - 1;
-const READERS_WAITING: i32 = 1 << 30;
-const WRITERS_WAITING: i32 = 1 << 31;
+const READ_LOCKED: u32 = 1;
+const MASK: u32 = (1 << 30) - 1;
+const WRITE_LOCKED: u32 = MASK;
+const MAX_READERS: u32 = MASK - 1;
+const READERS_WAITING: u32 = 1 << 30;
+const WRITERS_WAITING: u32 = 1 << 31;
 
-fn is_unlocked(state: i32) -> bool {
+fn is_unlocked(state: u32) -> bool {
     state & MASK == 0
 }
 
-fn is_write_locked(state: i32) -> bool {
+fn is_write_locked(state: u32) -> bool {
     state & MASK == WRITE_LOCKED
 }
 
-fn has_readers_waiting(state: i32) -> bool {
+fn has_readers_waiting(state: u32) -> bool {
     state & READERS_WAITING != 0
 }
 
-fn has_writers_waiting(state: i32) -> bool {
+fn has_writers_waiting(state: u32) -> bool {
     state & WRITERS_WAITING != 0
 }
 
-fn is_read_lockable(state: i32) -> bool {
+fn is_read_lockable(state: u32) -> bool {
     // This also returns false if the counter could overflow if we tried to read lock it.
     //
     // We don't allow read-locking if there's readers waiting, even if the lock is unlocked
@@ -53,14 +53,14 @@ fn is_read_lockable(state: i32) -> bool {
     state & MASK < MAX_READERS && !has_readers_waiting(state) && !has_writers_waiting(state)
 }
 
-fn has_reached_max_readers(state: i32) -> bool {
+fn has_reached_max_readers(state: u32) -> bool {
     state & MASK == MAX_READERS
 }
 
 impl RwLock {
     #[inline]
     pub const fn new() -> Self {
-        Self { state: AtomicI32::new(0), writer_notify: AtomicI32::new(0) }
+        Self { state: AtomicU32::new(0), writer_notify: AtomicU32::new(0) }
     }
 
     #[inline]
@@ -227,7 +227,7 @@ impl RwLock {
     /// If both are waiting, this will wake up only one writer, but will fall
     /// back to waking up readers if there was no writer to wake up.
     #[cold]
-    fn wake_writer_or_readers(&self, mut state: i32) {
+    fn wake_writer_or_readers(&self, mut state: u32) {
         assert!(is_unlocked(state));
 
         // The readers waiting bit might be turned on at any point now,
@@ -287,7 +287,7 @@ impl RwLock {
     }
 
     /// Spin for a while, but stop directly at the given condition.
-    fn spin_until(&self, f: impl Fn(i32) -> bool) -> i32 {
+    fn spin_until(&self, f: impl Fn(u32) -> bool) -> u32 {
         let mut spin = 100; // Chosen by fair dice roll.
         loop {
             let state = self.state.load(Relaxed);
@@ -299,12 +299,12 @@ impl RwLock {
         }
     }
 
-    fn spin_write(&self) -> i32 {
+    fn spin_write(&self) -> u32 {
         // Stop spinning when it's unlocked or when there's waiting writers, to keep things somewhat fair.
         self.spin_until(|state| is_unlocked(state) || has_writers_waiting(state))
     }
 
-    fn spin_read(&self) -> i32 {
+    fn spin_read(&self) -> u32 {
         // Stop spinning when it's unlocked or read locked, or when there's waiting threads.
         self.spin_until(|state| {
             !is_write_locked(state) || has_readers_waiting(state) || has_writers_waiting(state)

library/std/src/sys/wasm/atomics/futex.rs

@@ -1,17 +1,21 @@
 use crate::arch::wasm32;
 use crate::convert::TryInto;
-use crate::sync::atomic::AtomicI32;
+use crate::sync::atomic::AtomicU32;
 use crate::time::Duration;
 
-pub fn futex_wait(futex: &AtomicI32, expected: i32, timeout: Option<Duration>) {
+pub fn futex_wait(futex: &AtomicU32, expected: u32, timeout: Option<Duration>) {
     let timeout = timeout.and_then(|t| t.as_nanos().try_into().ok()).unwrap_or(-1);
     unsafe {
-        wasm32::memory_atomic_wait32(futex as *const AtomicI32 as *mut i32, expected, timeout);
+        wasm32::memory_atomic_wait32(
+            futex as *const AtomicU32 as *mut i32,
+            expected as i32,
+            timeout,
+        );
     }
 }
 
-pub fn futex_wake(futex: &AtomicI32) {
+pub fn futex_wake(futex: &AtomicU32) {
     unsafe {
-        wasm32::memory_atomic_notify(futex as *const AtomicI32 as *mut i32, 1);
+        wasm32::memory_atomic_notify(futex as *const AtomicU32 as *mut i32, 1);
     }
 }

library/std/src/sys_common/thread_parker/futex.rs

@@ -1,14 +1,14 @@
-use crate::sync::atomic::AtomicI32;
+use crate::sync::atomic::AtomicU32;
 use crate::sync::atomic::Ordering::{Acquire, Release};
 use crate::sys::futex::{futex_wait, futex_wake};
 use crate::time::Duration;
 
-const PARKED: i32 = -1;
-const EMPTY: i32 = 0;
-const NOTIFIED: i32 = 1;
+const PARKED: u32 = u32::MAX;
+const EMPTY: u32 = 0;
+const NOTIFIED: u32 = 1;
 
 pub struct Parker {
-    state: AtomicI32,
+    state: AtomicU32,
 }
 
 // Notes about memory ordering:
@@ -34,7 +34,7 @@ pub struct Parker {
 impl Parker {
     #[inline]
     pub const fn new() -> Self {
-        Parker { state: AtomicI32::new(EMPTY) }
+        Parker { state: AtomicU32::new(EMPTY) }
     }
 
     // Assumes this is only called by the thread that owns the Parker,