rt: Allow concurrent block_on's with basic_scheduler

This allows us to concurrently call `Runtime::block_on` with the basic_scheduler
and allowing other threads to steal the dedicated parker.

tokio/src/park/thread.rs

@@ -21,8 +21,8 @@ pub(crate) struct UnparkThread {
 #[derive(Debug)]
 struct Inner {
     state: AtomicUsize,
-    mutex: Mutex<()>,
-    condvar: Condvar,
+    mutex: Arc<Mutex<()>>,
+    condvar: Arc<Condvar>,
 }
 
 const EMPTY: usize = 0;
@@ -37,11 +37,15 @@ thread_local! {
 
 impl ParkThread {
     pub(crate) fn new() -> Self {
+        ParkThread::with_condvar(Arc::new(Condvar::new()), Arc::new(Mutex::new(())))
+    }
+
+    pub(crate) fn with_condvar(condvar: Arc<Condvar>, mutex: Arc<Mutex<()>>) -> Self {
         Self {
             inner: Arc::new(Inner {
                 state: AtomicUsize::new(EMPTY),
-                mutex: Mutex::new(()),
-                condvar: Condvar::new(),
+                mutex,
+                condvar,
             }),
         }
     }

tokio/src/runtime/basic_scheduler.rs

@@ -1,22 +1,35 @@
-use crate::park::{Park, Unpark};
+use crate::loom;
+use crate::loom::sync::{Condvar, Mutex};
+use crate::park::{Park, ParkThread, Unpark};
 use crate::runtime;
 use crate::runtime::task::{self, JoinHandle, Schedule, Task};
 use crate::util::linked_list::LinkedList;
-use crate::util::{waker_ref, Wake};
+use crate::util::{waker_ref, Wake, WakerRef};
 
 use std::cell::RefCell;
 use std::collections::VecDeque;
 use std::fmt;
 use std::future::Future;
-use std::sync::{Arc, Mutex};
+use std::sync::Arc;
 use std::task::Poll::Ready;
 use std::time::Duration;
 
 /// Executes tasks on the current thread
-pub(crate) struct BasicScheduler<P>
-where
-    P: Park,
-{
+pub(crate) struct BasicScheduler<P: Park> {
+    /// Inner with the dedicated parker P
+    inner: Mutex<Option<Inner<P>>>,
+
+    /// Sync items used to notify other threads that called
+    /// block_on concurrently that the dedicated driver is available
+    /// to steal
+    condvar: loom::sync::Arc<Condvar>,
+    mutex: loom::sync::Arc<Mutex<()>>,
+
+    /// Sendable task spawner
+    spawner: Spawner,
+}
+
+struct Inner<P: Park> {
     /// Scheduler run queue
     ///
     /// When the scheduler is executed, the queue is removed from `self` and
@@ -59,7 +72,7 @@ struct Shared {
     unpark: Box<dyn Unpark>,
 }
 
-/// Thread-local context
+/// Thread-local context.
 struct Context {
     /// Shared scheduler state
     shared: Arc<Shared>,
@@ -68,16 +81,16 @@ struct Context {
     tasks: RefCell<Tasks>,
 }
 
-/// Initial queue capacity
+/// Initial queue capacity.
 const INITIAL_CAPACITY: usize = 64;
 
 /// Max number of tasks to poll per tick.
 const MAX_TASKS_PER_TICK: usize = 61;
 
-/// How often ot check the remote queue first
+/// How often to check the remote queue first.
 const REMOTE_FIRST_INTERVAL: u8 = 31;
 
-// Tracks the current BasicScheduler
+// Tracks the current BasicScheduler.
 scoped_thread_local!(static CURRENT: Context);
 
 impl<P> BasicScheduler<P>
@@ -87,19 +100,28 @@ where
     pub(crate) fn new(park: P) -> BasicScheduler<P> {
         let unpark = Box::new(park.unpark());
 
-        BasicScheduler {
+        let spawner = Spawner {
+            shared: Arc::new(Shared {
+                queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
+                unpark: unpark as Box<dyn Unpark>,
+            }),
+        };
+
+        let inner = Mutex::new(Some(Inner {
             tasks: Some(Tasks {
                 owned: LinkedList::new(),
                 queue: VecDeque::with_capacity(INITIAL_CAPACITY),
             }),
-            spawner: Spawner {
-                shared: Arc::new(Shared {
-                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
-                    unpark: unpark as Box<dyn Unpark>,
-                }),
-            },
+            spawner: spawner.clone(),
             tick: 0,
             park,
+        }));
+
+        BasicScheduler {
+            inner,
+            spawner,
+            condvar: loom::sync::Arc::new(Condvar::new()),
+            mutex: loom::sync::Arc::new(Mutex::new(())),
         }
     }
 
@@ -108,7 +130,6 @@ where
     }
 
     /// Spawns a future onto the thread pool
-    #[allow(dead_code)]
     pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
     where
         F: Future + Send + 'static,
@@ -117,13 +138,60 @@ where
         self.spawner.spawn(future)
     }
 
-    pub(crate) fn block_on<F>(&mut self, future: F) -> F::Output
-    where
-        F: Future,
-    {
+    pub(crate) fn block_on<F: Future>(&self, future: F) -> F::Output {
+        // If we can steal the dedicated parker than lets block_on that
+        // otherwise, lets block_on and attempt to steal it back if we can.
+        if let Some(mut inner) = self.take_inner() {
+            inner.block_on(future)
+        } else {
+            // TODO: should this be false or true? In the origina block_on for
+            // basic_scheduler we have false?
+            let enter = crate::runtime::enter(false);
+
+            let mut park = ParkThread::with_condvar(self.condvar.clone(), self.mutex.clone());
+            let waker = park.unpark().into_waker();
+            let mut cx = std::task::Context::from_waker(&waker);
+
+            pin!(future);
+
+            loop {
+                if let Ready(v) = crate::coop::budget(|| future.as_mut().poll(&mut cx)) {
+                    return v;
+                }
+
+                // Check if we can steal the driver
+                // TODO: Consider using an atomic load here intead of locking
+                // the mutex.
+                if let Some(mut inner) = self.take_inner() {
+                    // We will enter again on in the inner implementation below
+                    drop(enter);
+                    return inner.block_on(future);
+                }
+
+                // Park this thread waiting for some external wake up from
+                // a waker or a notification that we can steal the driver again.
+                park.park().expect("failed to park");
+            }
+        }
+    }
+
+    fn take_inner(&self) -> Option<InnerGuard<'_, P>> {
+        let mut lock = self.inner.lock().unwrap();
+        let inner = lock.take()?;
+
+        Some(InnerGuard {
+            inner: Some(inner),
+            scheduler: &self,
+        })
+    }
+}
+
+impl<P: Park> Inner<P> {
+    /// Block on the future provided and drive the runtime's driver.
+    fn block_on<F: Future>(&mut self, future: F) -> F::Output {
         enter(self, |scheduler, context| {
             let _enter = runtime::enter(false);
-            let waker = waker_ref(&scheduler.spawner.shared);
+            let waker = scheduler.spawner.waker_ref();
             let mut cx = std::task::Context::from_waker(&waker);
 
             pin!(future);
@@ -178,16 +246,16 @@ where
 
 /// Enter the scheduler context. This sets the queue and other necessary
 /// scheduler state in the thread-local
-fn enter<F, R, P>(scheduler: &mut BasicScheduler<P>, f: F) -> R
+fn enter<F, R, P>(scheduler: &mut Inner<P>, f: F) -> R
 where
-    F: FnOnce(&mut BasicScheduler<P>, &Context) -> R,
+    F: FnOnce(&mut Inner<P>, &Context) -> R,
     P: Park,
 {
     // Ensures the run queue is placed back in the `BasicScheduler` instance
     // once `block_on` returns.`
     struct Guard<'a, P: Park> {
         context: Option<Context>,
-        scheduler: &'a mut BasicScheduler<P>,
+        scheduler: &'a mut Inner<P>,
     }
 
     impl<P: Park> Drop for Guard<'_, P> {
@@ -214,12 +282,15 @@ where
     CURRENT.set(context, || f(scheduler, context))
 }
 
-impl<P> Drop for BasicScheduler<P>
-where
-    P: Park,
-{
+impl<P: Park> Drop for BasicScheduler<P> {
     fn drop(&mut self) {
-        enter(self, |scheduler, context| {
+        let mut inner = {
+            let mut lock = self.inner.lock().expect("BasicScheduler Inner lock");
+            lock.take()
+                .expect("Oh no! We never placed the Inner state back!")
+        };
+
+        enter(&mut inner, |scheduler, context| {
             // Loop required here to ensure borrow is dropped between iterations
             #[allow(clippy::while_let_loop)]
             loop {
@@ -269,6 +340,10 @@ impl Spawner {
     fn pop(&self) -> Option<task::Notified<Arc<Shared>>> {
         self.shared.queue.lock().unwrap().pop_front()
     }
+
+    fn waker_ref(&self) -> WakerRef<'_> {
+        waker_ref(&self.shared)
+    }
 }
 
 impl fmt::Debug for Spawner {
@@ -325,3 +400,36 @@ impl Wake for Shared {
         arc_self.unpark.unpark();
     }
 }
+
+// ===== InnerGuard =====
+
+/// Used to ensure we always place the Inner value
+/// back into its slot in `BasicScheduler` even if the
+/// future panics.
+struct InnerGuard<'a, P: Park> {
+    inner: Option<Inner<P>>,
+    scheduler: &'a BasicScheduler<P>,
+}
+
+impl<P: Park> InnerGuard<'_, P> {
+    fn block_on<F: Future>(&mut self, future: F) -> F::Output {
+        // The only time inner gets set to `None` is if we have dropped
+        // already so this unwrap is safe.
+        self.inner.as_mut().unwrap().block_on(future)
+    }
+}
+
+impl<P: Park> Drop for InnerGuard<'_, P> {
+    fn drop(&mut self) {
+        if let Some(inner) = self.inner.take() {
+            let mut lock = self.scheduler.inner.lock().unwrap();
+            lock.replace(inner);
+
+            // Wake up possible other threads
+            // notifying them that they might need
+            // to steal the driver.
+            drop(self.scheduler.mutex.lock().unwrap());
+            self.scheduler.condvar.notify_one();
+        }
+    }
+}

tokio/src/runtime/builder.rs

@@ -463,7 +463,7 @@ cfg_rt_core! {
             let blocking_spawner = blocking_pool.spawner().clone();
 
             Ok(Runtime {
-                kind: Kind::Basic(Mutex::new(Some(scheduler))),
+                kind: Kind::Basic(scheduler),
                 handle: Handle {
                     spawner,
                     io_handle,

tokio/src/runtime/mod.rs

@@ -293,7 +293,7 @@ enum Kind {
 
     /// Execute all tasks on the current-thread.
     #[cfg(feature = "rt-core")]
-    Basic(Mutex<Option<BasicScheduler<time::Driver>>>),
+    Basic(BasicScheduler<time::Driver>),
 
     /// Execute tasks across multiple threads.
     #[cfg(feature = "rt-threaded")]
@@ -398,7 +398,7 @@ impl Runtime {
             Kind::Shell(_) => panic!("task execution disabled"),
             #[cfg(feature = "rt-threaded")]
             Kind::ThreadPool(exec) => exec.spawn(future),
-            Kind::Basic(_exec) => self.handle.spawner.spawn(future),
+            Kind::Basic(exec) => exec.spawn(future),
         }
     }
 
@@ -458,24 +458,7 @@ impl Runtime {
                 }
             }
             #[cfg(feature = "rt-core")]
-            Kind::Basic(exec) => {
-                // TODO(lucio): clean this up and move this impl into
-                // `basic_scheduler.rs`, this is hacky and bad but will work for
-                // now.
-                let exec_temp = {
-                    let mut lock = exec.lock().unwrap();
-                    lock.take()
-                };
-
-                if let Some(mut exec_temp) = exec_temp {
-                    let res = exec_temp.block_on(future);
-                    exec.lock().unwrap().replace(exec_temp);
-                    res
-                } else {
-                    let mut enter = crate::runtime::enter(true);
-                    enter.block_on(future).unwrap()
-                }
-            }
+            Kind::Basic(exec) => exec.block_on(future),
             #[cfg(feature = "rt-threaded")]
             Kind::ThreadPool(exec) => exec.block_on(future),
         })

tokio/src/util/mod.rs

@@ -12,6 +12,10 @@ mod rand;
 mod wake;
 pub(crate) use wake::{waker_ref, Wake};
 
+cfg_rt_core! {
+    pub(crate) use wake::WakerRef;
+}
+
 cfg_rt_threaded! {
     pub(crate) use rand::FastRand;