std: replace generic thread parker with explicit no-op parker

library/std/src/sys/unsupported/mod.rs

@@ -22,6 +22,7 @@ pub mod thread;
 #[cfg(target_thread_local)]
 pub mod thread_local_dtor;
 pub mod thread_local_key;
+pub mod thread_parking;
 pub mod time;
 
 mod common;

library/std/src/sys/unsupported/thread_parking.rs

@@ -0,0 +1,11 @@
+use crate::pin::Pin;
+use crate::time::Duration;
+
+pub struct Parker {}
+
+impl Parker {
+    pub unsafe fn new_in_place(_parker: *mut Parker) {}
+    pub unsafe fn park(self: Pin<&Self>) {}
+    pub unsafe fn park_timeout(self: Pin<&Self>, _dur: Duration) {}
+    pub fn unpark(self: Pin<&Self>) {}
+}

library/std/src/sys/wasi/mod.rs

@@ -49,6 +49,8 @@ pub mod thread;
 pub mod thread_local_dtor;
 #[path = "../unsupported/thread_local_key.rs"]
 pub mod thread_local_key;
+#[path = "../unsupported/thread_parking.rs"]
+pub mod thread_parking;
 pub mod time;
 
 #[path = "../unsupported/common.rs"]

library/std/src/sys/wasm/mod.rs

@@ -70,6 +70,8 @@ cfg_if::cfg_if! {
         pub mod once;
         #[path = "../unsupported/thread.rs"]
         pub mod thread;
+        #[path = "../unsupported/thread_parking.rs"]
+        pub mod thread_parking;
     }
 }
 

library/std/src/sys_common/thread_parking/generic.rs

@@ -1,125 +0,0 @@
-//! Parker implementation based on a Mutex and Condvar.
-
-use crate::pin::Pin;
-use crate::sync::atomic::AtomicUsize;
-use crate::sync::atomic::Ordering::SeqCst;
-use crate::sync::{Condvar, Mutex};
-use crate::time::Duration;
-
-const EMPTY: usize = 0;
-const PARKED: usize = 1;
-const NOTIFIED: usize = 2;
-
-pub struct Parker {
-    state: AtomicUsize,
-    lock: Mutex<()>,
-    cvar: Condvar,
-}
-
-impl Parker {
-    /// Construct the generic parker. The UNIX parker implementation
-    /// requires this to happen in-place.
-    pub unsafe fn new_in_place(parker: *mut Parker) {
-        parker.write(Parker {
-            state: AtomicUsize::new(EMPTY),
-            lock: Mutex::new(()),
-            cvar: Condvar::new(),
-        });
-    }
-
-    // This implementation doesn't require `unsafe` and `Pin`, but other implementations do.
-    pub unsafe fn park(self: Pin<&Self>) {
-        // If we were previously notified then we consume this notification and
-        // return quickly.
-        if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
-            return;
-        }
-
-        // Otherwise we need to coordinate going to sleep
-        let mut m = self.lock.lock().unwrap();
-        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
-            Ok(_) => {}
-            Err(NOTIFIED) => {
-                // We must read here, even though we know it will be `NOTIFIED`.
-                // This is because `unpark` may have been called again since we read
-                // `NOTIFIED` in the `compare_exchange` above. We must perform an
-                // acquire operation that synchronizes with that `unpark` to observe
-                // any writes it made before the call to unpark. To do that we must
-                // read from the write it made to `state`.
-                let old = self.state.swap(EMPTY, SeqCst);
-                assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
-                return;
-            } // should consume this notification, so prohibit spurious wakeups in next park.
-            Err(_) => panic!("inconsistent park state"),
-        }
-        loop {
-            m = self.cvar.wait(m).unwrap();
-            match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
-                Ok(_) => return, // got a notification
-                Err(_) => {}     // spurious wakeup, go back to sleep
-            }
-        }
-    }
-
-    // This implementation doesn't require `unsafe` and `Pin`, but other implementations do.
-    pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) {
-        // Like `park` above we have a fast path for an already-notified thread, and
-        // afterwards we start coordinating for a sleep.
-        // return quickly.
-        if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
-            return;
-        }
-        let m = self.lock.lock().unwrap();
-        match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
-            Ok(_) => {}
-            Err(NOTIFIED) => {
-                // We must read again here, see `park`.
-                let old = self.state.swap(EMPTY, SeqCst);
-                assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
-                return;
-            } // should consume this notification, so prohibit spurious wakeups in next park.
-            Err(_) => panic!("inconsistent park_timeout state"),
-        }
-
-        // Wait with a timeout, and if we spuriously wake up or otherwise wake up
-        // from a notification we just want to unconditionally set the state back to
-        // empty, either consuming a notification or un-flagging ourselves as
-        // parked.
-        let (_m, _result) = self.cvar.wait_timeout(m, dur).unwrap();
-        match self.state.swap(EMPTY, SeqCst) {
-            NOTIFIED => {} // got a notification, hurray!
-            PARKED => {}   // no notification, alas
-            n => panic!("inconsistent park_timeout state: {n}"),
-        }
-    }
-
-    // This implementation doesn't require `Pin`, but other implementations do.
-    pub fn unpark(self: Pin<&Self>) {
-        // To ensure the unparked thread will observe any writes we made
-        // before this call, we must perform a release operation that `park`
-        // can synchronize with. To do that we must write `NOTIFIED` even if
-        // `state` is already `NOTIFIED`. That is why this must be a swap
-        // rather than a compare-and-swap that returns if it reads `NOTIFIED`
-        // on failure.
-        match self.state.swap(NOTIFIED, SeqCst) {
-            EMPTY => return,    // no one was waiting
-            NOTIFIED => return, // already unparked
-            PARKED => {}        // gotta go wake someone up
-            _ => panic!("inconsistent state in unpark"),
-        }
-
-        // There is a period between when the parked thread sets `state` to
-        // `PARKED` (or last checked `state` in the case of a spurious wake
-        // up) and when it actually waits on `cvar`. If we were to notify
-        // during this period it would be ignored and then when the parked
-        // thread went to sleep it would never wake up. Fortunately, it has
-        // `lock` locked at this stage so we can acquire `lock` to wait until
-        // it is ready to receive the notification.
-        //
-        // Releasing `lock` before the call to `notify_one` means that when the
-        // parked thread wakes it doesn't get woken only to have to wait for us
-        // to release `lock`.
-        drop(self.lock.lock().unwrap());
-        self.cvar.notify_one()
-    }
-}

library/std/src/sys_common/thread_parking/mod.rs

@@ -18,10 +18,7 @@ cfg_if::cfg_if! {
     ))] {
         mod id;
         pub use id::Parker;
-    } else if #[cfg(any(windows, target_family = "unix"))] {
-        pub use crate::sys::thread_parking::Parker;
     } else {
-        mod generic;
-        pub use generic::Parker;
+        pub use crate::sys::thread_parking::Parker;
     }
 }