Auto merge of #45524 - alexcrichton:improve-park-unpark, r=dtolnay

std: Optimize thread park/unpark implementation

This is an adaptation of rust-lang/futures-rs#597 for the standard library.
The goal here is to avoid locking a mutex on the "fast path" for thread
park/unpark where you're waking up a thread that isn't sleeping or otherwise
trying to park a thread that's already been notified. Mutex performance varies
quite a bit across platforms so this should provide a nice consistent speed
boost for the fast path of these functions.

src/libstd/thread/mod.rs

@@ -171,6 +171,8 @@ use panic;
 use panicking;
 use str;
 use sync::{Mutex, Condvar, Arc};
+use sync::atomic::AtomicUsize;
+use sync::atomic::Ordering::SeqCst;
 use sys::thread as imp;
 use sys_common::mutex;
 use sys_common::thread_info;
@@ -694,6 +696,11 @@ pub fn sleep(dur: Duration) {
     imp::Thread::sleep(dur)
 }
 
+// constants for park/unpark
+const EMPTY: usize = 0;
+const PARKED: usize = 1;
+const NOTIFIED: usize = 2;
+
 /// Blocks unless or until the current thread's token is made available.
 ///
 /// A call to `park` does not guarantee that the thread will remain parked
@@ -771,11 +778,27 @@ pub fn sleep(dur: Duration) {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn park() {
     let thread = current();
-    let mut guard = thread.inner.lock.lock().unwrap();
-    while !*guard {
-        guard = thread.inner.cvar.wait(guard).unwrap();
+
+    // If we were previously notified then we consume this notification and
+    // return quickly.
+    if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
+        return
+    }
+
+    // Otherwise we need to coordinate going to sleep
+    let mut m = thread.inner.lock.lock().unwrap();
+    match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+        Ok(_) => {}
+        Err(NOTIFIED) => return, // notified after we locked
+        Err(_) => panic!("inconsistent park state"),
+    }
+    loop {
+        m = thread.inner.cvar.wait(m).unwrap();
+        match thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
+            Ok(_) => return, // got a notification
+            Err(_) => {} // spurious wakeup, go back to sleep
+        }
     }
-    *guard = false;
 }
 
 /// Use [`park_timeout`].
@@ -842,12 +865,30 @@ pub fn park_timeout_ms(ms: u32) {
 #[stable(feature = "park_timeout", since = "1.4.0")]
 pub fn park_timeout(dur: Duration) {
     let thread = current();
-    let mut guard = thread.inner.lock.lock().unwrap();
-    if !*guard {
-        let (g, _) = thread.inner.cvar.wait_timeout(guard, dur).unwrap();
-        guard = g;
+
+    // Like `park` above we have a fast path for an already-notified thread, and
+    // afterwards we start coordinating for a sleep.
+    // return quickly.
+    if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
+        return
+    }
+    let m = thread.inner.lock.lock().unwrap();
+    match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
+        Ok(_) => {}
+        Err(NOTIFIED) => return, // notified after we locked
+        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) = thread.inner.cvar.wait_timeout(m, dur).unwrap();
+    match thread.inner.state.swap(EMPTY, SeqCst) {
+        NOTIFIED => {} // got a notification, hurray!
+        PARKED => {} // no notification, alas
+        n => panic!("inconsistent park_timeout state: {}", n),
     }
-    *guard = false;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -914,7 +955,10 @@ impl ThreadId {
 struct Inner {
     name: Option<CString>,      // Guaranteed to be UTF-8
     id: ThreadId,
-    lock: Mutex<bool>,          // true when there is a buffered unpark
+
+    // state for thread park/unpark
+    state: AtomicUsize,
+    lock: Mutex<()>,
     cvar: Condvar,
 }
 
@@ -958,7 +1002,8 @@ impl Thread {
             inner: Arc::new(Inner {
                 name: cname,
                 id: ThreadId::new(),
-                lock: Mutex::new(false),
+                state: AtomicUsize::new(EMPTY),
+                lock: Mutex::new(()),
                 cvar: Condvar::new(),
             })
         }
@@ -998,10 +1043,22 @@ impl Thread {
     /// [park]: fn.park.html
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn unpark(&self) {
-        let mut guard = self.inner.lock.lock().unwrap();
-        if !*guard {
-            *guard = true;
-            self.inner.cvar.notify_one();
+        loop {
+            match self.inner.state.compare_exchange(EMPTY, NOTIFIED, SeqCst, SeqCst) {
+                Ok(_) => return, // no one was waiting
+                Err(NOTIFIED) => return, // already unparked
+                Err(PARKED) => {} // gotta go wake someone up
+                _ => panic!("inconsistent state in unpark"),
+            }
+
+            // Coordinate wakeup through the mutex and a condvar notification
+            let _lock = self.inner.lock.lock().unwrap();
+            match self.inner.state.compare_exchange(PARKED, NOTIFIED, SeqCst, SeqCst) {
+                Ok(_) => return self.inner.cvar.notify_one(),
+                Err(NOTIFIED) => return, // a different thread unparked
+                Err(EMPTY) => {} // parked thread went away, try again
+                _ => panic!("inconsistent state in unpark"),
+            }
         }
     }