make Miri's scheduler proper round-robin

src/thread.rs

@@ -518,16 +518,26 @@ impl<'mir, 'tcx: 'mir> ThreadManager<'mir, 'tcx> {
             return Ok(SchedulingAction::ExecuteTimeoutCallback);
         }
         // No callbacks scheduled, pick a regular thread to execute.
-        // We need to pick a new thread for execution.
-        for (id, thread) in self.threads.iter_enumerated() {
+        // The active thread blocked or yielded. So we go search for another enabled thread.
+        // Curcially, we start searching at the current active thread ID, rather than at 0, since we
+        // want to avoid always scheduling threads 0 and 1 without ever making progress in thread 2.
+        //
+        // `skip(N)` means we start iterating at thread N, so we skip 1 more to start just *after*
+        // the active thread. Then after that we look at `take(N)`, i.e., the threads *before* the
+        // active thread.
+        let threads = self
+            .threads
+            .iter_enumerated()
+            .skip(self.active_thread.index() + 1)
+            .chain(self.threads.iter_enumerated().take(self.active_thread.index()));
+        for (id, thread) in threads {
+            debug_assert_ne!(self.active_thread, id);
             if thread.state == ThreadState::Enabled {
-                if !self.yield_active_thread || id != self.active_thread {
-                    self.active_thread = id;
-                    if let Some(data_race) = data_race {
-                        data_race.thread_set_active(self.active_thread);
-                    }
-                    break;
+                self.active_thread = id;
+                if let Some(data_race) = data_race {
+                    data_race.thread_set_active(self.active_thread);
                 }
+                break;
             }
         }
         self.yield_active_thread = false;

tests/pass/concurrency/spin_loops.rs

@@ -0,0 +1,82 @@
+// ignore-windows: Concurrency on Windows is not supported yet.
+
+use std::thread;
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::mpsc;
+use std::cell::Cell;
+
+/// When a thread yields, Miri's scheduler used to pick the thread with the lowest ID
+/// that can run. IDs are assigned in thread creation order.
+/// This means we could make 2 threads infinitely ping-pong with each other while
+/// really there is a 3rd thread that we should schedule to make progress.
+fn two_player_ping_pong() {
+    static FLAG: AtomicUsize = AtomicUsize::new(0);
+
+    let waiter1 = thread::spawn(|| {
+        while FLAG.load(Ordering::Acquire) == 0 {
+            // spin and wait
+            thread::yield_now();
+        }
+    });
+    let waiter2 = thread::spawn(|| {
+        while FLAG.load(Ordering::Acquire) == 0 {
+            // spin and wait
+            thread::yield_now();
+        }
+    });
+    let progress = thread::spawn(|| {
+        FLAG.store(1, Ordering::Release);
+    });
+    // The first `join` blocks the main thread and thus takes it out of the equation.
+    waiter1.join().unwrap();
+    waiter2.join().unwrap();
+    progress.join().unwrap();
+}
+
+/// Based on a test by @jethrogb.
+fn launcher() {
+    static THREAD2_LAUNCHED: AtomicBool = AtomicBool::new(false);
+
+    for _ in 0..10 {
+        let (tx, rx) = mpsc::sync_channel(0);
+        THREAD2_LAUNCHED.store(false, Ordering::SeqCst);
+
+        let jh = thread::spawn(move || {
+            struct RecvOnDrop(Cell<Option<mpsc::Receiver<()>>>);
+
+            impl Drop for RecvOnDrop {
+                fn drop(&mut self) {
+                    let rx = self.0.take().unwrap();
+                    while !THREAD2_LAUNCHED.load(Ordering::SeqCst) {
+                        thread::yield_now();
+                    }
+                    rx.recv().unwrap();
+                }
+            }
+
+            let tl_rx: RecvOnDrop = RecvOnDrop(Cell::new(None));
+            tl_rx.0.set(Some(rx));
+        });
+
+        let tx_clone = tx.clone();
+        let jh2 = thread::spawn(move || {
+            THREAD2_LAUNCHED.store(true, Ordering::SeqCst);
+            jh.join().unwrap();
+            tx_clone.send(()).expect_err(
+                "Expecting channel to be closed because thread 1 TLS destructors must've run",
+            );
+        });
+
+        while !THREAD2_LAUNCHED.load(Ordering::SeqCst) {
+            thread::yield_now();
+        }
+        thread::yield_now();
+        tx.send(()).expect("Expecting channel to be live because thread 2 must block on join");
+        jh2.join().unwrap();
+    }
+}
+
+fn main() {
+    two_player_ping_pong();
+    launcher();
+}

tests/pass/concurrency/spin_loops.stderr

@@ -0,0 +1,2 @@
+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
+