diff --git a/src/rt/runtime.rs b/src/rt/runtime.rs
index 3a08bb6..8f84f54 100644
--- a/src/rt/runtime.rs
+++ b/src/rt/runtime.rs
@@ -2,7 +2,7 @@ use std::cell::Cell;
 use std::io;
 use std::iter;
 use std::sync::atomic::{self, Ordering};
-use std::sync::Arc;
+use std::sync::{Arc, Mutex};
 use std::thread;
 use std::time::Duration;
 
@@ -22,6 +22,11 @@ thread_local! {
     static YIELD_NOW: Cell<bool> = Cell::new(false);
 }
 
+struct Scheduler {
+    /// Set to `true` while a machine is polling the reactor.
+    polling: bool,
+}
+
 /// An async runtime.
 pub struct Runtime {
     /// The reactor.
@@ -33,7 +38,11 @@ pub struct Runtime {
     /// Handles to local queues for stealing work.
     stealers: Vec<Stealer<Runnable>>,
 
+    /// Machines to start
     machines: Vec<Arc<Machine>>,
+
+    /// The scheduler state.
+    sched: Mutex<Scheduler>,
 }
 
 impl Runtime {
@@ -57,6 +66,7 @@ impl Runtime {
             injector: Injector::new(),
             stealers,
             machines,
+            sched: Mutex::new(Scheduler { polling: false }),
         }
     }
 
@@ -116,7 +126,25 @@ impl Runtime {
     /// This function might not poll the reactor at all so do not rely on it doing anything. Only
     /// use for optimization.
     fn quick_poll(&self) -> io::Result<bool> {
-        return self.reactor.poll(Some(Duration::from_secs(0)));
+        if let Ok(sched) = self.sched.try_lock() {
+            if !sched.polling {
+                return self.reactor.poll(Some(Duration::from_secs(0)));
+            }
+        }
+        Ok(false)
+    }
+
+    fn poll(&self) -> io::Result<bool> {
+        let mut sched = self.sched.lock().unwrap();
+        sched.polling = true;
+        drop(sched);
+
+        let result = self.reactor.poll(None);
+
+        let mut sched = self.sched.lock().unwrap();
+        sched.polling = false;
+
+        result
     }
 }
 
@@ -242,7 +270,7 @@ impl Machine {
                 continue;
             }
 
-            rt.reactor.poll(None).unwrap();
+            rt.poll().unwrap();
 
             runs = 0;
             fails = 0;