rt: use optional non-zero value for task owner_id (#5876)

We switch to using a `NonZeroU64` for the `id` field for `OwnedTasks`
and `LocalOwnedTasks` lists. This allows the task header to contain an
`Option<NonZeroU64>` instead of a `u64` with a special meaning for 0.

The size in memory will be the same thanks to Rust's niche optimization,
but this solution is clearer in its intent.

Co-authored-by: Alice Ryhl <[email protected]>

tokio/src/runtime/task/core.rs

@@ -17,6 +17,7 @@ use crate::runtime::task::state::State;
 use crate::runtime::task::{Id, Schedule};
 use crate::util::linked_list;
 
+use std::num::NonZeroU64;
 use std::pin::Pin;
 use std::ptr::NonNull;
 use std::task::{Context, Poll, Waker};
@@ -164,7 +165,7 @@ pub(crate) struct Header {
 
     /// This integer contains the id of the OwnedTasks or LocalOwnedTasks that
     /// this task is stored in. If the task is not in any list, should be the
-    /// id of the list that it was previously in, or zero if it has never been
+    /// id of the list that it was previously in, or `None` if it has never been
     /// in any list.
     ///
     /// Once a task has been bound to a list, it can never be bound to another
@@ -173,7 +174,7 @@ pub(crate) struct Header {
     /// The id is not unset when removed from a list because we want to be able
     /// to read the id without synchronization, even if it is concurrently being
     /// removed from the list.
-    pub(super) owner_id: UnsafeCell<u64>,
+    pub(super) owner_id: UnsafeCell<Option<NonZeroU64>>,
 
     /// The tracing ID for this instrumented task.
     #[cfg(all(tokio_unstable, feature = "tracing"))]
@@ -221,7 +222,7 @@ impl<T: Future, S: Schedule> Cell<T, S> {
                 state,
                 queue_next: UnsafeCell::new(None),
                 vtable,
-                owner_id: UnsafeCell::new(0),
+                owner_id: UnsafeCell::new(None),
                 #[cfg(all(tokio_unstable, feature = "tracing"))]
                 tracing_id,
             }
@@ -394,13 +395,13 @@ impl Header {
     }
 
     // safety: The caller must guarantee exclusive access to this field, and
-    // must ensure that the id is either 0 or the id of the OwnedTasks
+    // must ensure that the id is either `None` or the id of the OwnedTasks
     // containing this task.
-    pub(super) unsafe fn set_owner_id(&self, owner: u64) {
-        self.owner_id.with_mut(|ptr| *ptr = owner);
+    pub(super) unsafe fn set_owner_id(&self, owner: NonZeroU64) {
+        self.owner_id.with_mut(|ptr| *ptr = Some(owner));
     }
 
-    pub(super) fn get_owner_id(&self) -> u64 {
+    pub(super) fn get_owner_id(&self) -> Option<NonZeroU64> {
         // safety: If there are concurrent writes, then that write has violated
         // the safety requirements on `set_owner_id`.
         unsafe { self.owner_id.with(|ptr| *ptr) }

tokio/src/runtime/task/list.rs

@@ -13,10 +13,11 @@ use crate::runtime::task::{JoinHandle, LocalNotified, Notified, Schedule, Task};
 use crate::util::linked_list::{CountedLinkedList, Link, LinkedList};
 
 use std::marker::PhantomData;
+use std::num::NonZeroU64;
 
 // The id from the module below is used to verify whether a given task is stored
 // in this OwnedTasks, or some other task. The counter starts at one so we can
-// use zero for tasks not owned by any list.
+// use `None` for tasks not owned by any list.
 //
 // The safety checks in this file can technically be violated if the counter is
 // overflown, but the checks are not supposed to ever fail unless there is a
@@ -28,10 +29,10 @@ cfg_has_atomic_u64! {
 
     static NEXT_OWNED_TASKS_ID: AtomicU64 = AtomicU64::new(1);
 
-    fn get_next_id() -> u64 {
+    fn get_next_id() -> NonZeroU64 {
         loop {
             let id = NEXT_OWNED_TASKS_ID.fetch_add(1, Ordering::Relaxed);
-            if id != 0 {
+            if let Some(id) = NonZeroU64::new(id) {
                 return id;
             }
         }
@@ -43,11 +44,11 @@ cfg_not_has_atomic_u64! {
 
     static NEXT_OWNED_TASKS_ID: AtomicU32 = AtomicU32::new(1);
 
-    fn get_next_id() -> u64 {
+    fn get_next_id() -> NonZeroU64 {
         loop {
             let id = NEXT_OWNED_TASKS_ID.fetch_add(1, Ordering::Relaxed);
-            if id != 0 {
-                return u64::from(id);
+            if let Some(id) = NonZeroU64::new(u64::from(id)) {
+                return id;
             }
         }
     }
@@ -55,7 +56,7 @@ cfg_not_has_atomic_u64! {
 
 pub(crate) struct OwnedTasks<S: 'static> {
     inner: Mutex<CountedOwnedTasksInner<S>>,
-    id: u64,
+    id: NonZeroU64,
 }
 struct CountedOwnedTasksInner<S: 'static> {
     list: CountedLinkedList<Task<S>, <Task<S> as Link>::Target>,
@@ -63,7 +64,7 @@ struct CountedOwnedTasksInner<S: 'static> {
 }
 pub(crate) struct LocalOwnedTasks<S: 'static> {
     inner: UnsafeCell<OwnedTasksInner<S>>,
-    id: u64,
+    id: NonZeroU64,
     _not_send_or_sync: PhantomData<*const ()>,
 }
 struct OwnedTasksInner<S: 'static> {
@@ -127,7 +128,7 @@ impl<S: 'static> OwnedTasks<S> {
     /// a LocalNotified, giving the thread permission to poll this task.
     #[inline]
     pub(crate) fn assert_owner(&self, task: Notified<S>) -> LocalNotified<S> {
-        assert_eq!(task.header().get_owner_id(), self.id);
+        assert_eq!(task.header().get_owner_id(), Some(self.id));
 
         // safety: All tasks bound to this OwnedTasks are Send, so it is safe
         // to poll it on this thread no matter what thread we are on.
@@ -170,11 +171,9 @@ impl<S: 'static> OwnedTasks<S> {
     }
 
     pub(crate) fn remove(&self, task: &Task<S>) -> Option<Task<S>> {
-        let task_id = task.header().get_owner_id();
-        if task_id == 0 {
-            // The task is unowned.
-            return None;
-        }
+        // If the task's owner ID is `None` then it is not part of any list and
+        // doesn't need removing.
+        let task_id = task.header().get_owner_id()?;
 
         assert_eq!(task_id, self.id);
 
@@ -257,11 +256,9 @@ impl<S: 'static> LocalOwnedTasks<S> {
     }
 
     pub(crate) fn remove(&self, task: &Task<S>) -> Option<Task<S>> {
-        let task_id = task.header().get_owner_id();
-        if task_id == 0 {
-            // The task is unowned.
-            return None;
-        }
+        // If the task's owner ID is `None` then it is not part of any list and
+        // doesn't need removing.
+        let task_id = task.header().get_owner_id()?;
 
         assert_eq!(task_id, self.id);
 
@@ -275,7 +272,7 @@ impl<S: 'static> LocalOwnedTasks<S> {
     /// it to a LocalNotified, giving the thread permission to poll this task.
     #[inline]
     pub(crate) fn assert_owner(&self, task: Notified<S>) -> LocalNotified<S> {
-        assert_eq!(task.header().get_owner_id(), self.id);
+        assert_eq!(task.header().get_owner_id(), Some(self.id));
 
         // safety: The task was bound to this LocalOwnedTasks, and the
         // LocalOwnedTasks is not Send or Sync, so we are on the right thread
@@ -315,11 +312,9 @@ mod tests {
     #[test]
     fn test_id_not_broken() {
         let mut last_id = get_next_id();
-        assert_ne!(last_id, 0);
 
         for _ in 0..1000 {
             let next_id = get_next_id();
-            assert_ne!(next_id, 0);
             assert!(last_id < next_id);
             last_id = next_id;
         }