task: fix the incomplete/wrong description of JOIN_WAKER bit. (#5217)

* task: fix the incomplete/wrong description of JOIN_WAKER bit (#5217)

tokio/src/runtime/task/mod.rs

@@ -47,7 +47,8 @@
 //!
 //!  * JOIN_INTEREST - Is set to one if there exists a JoinHandle.
 //!
-//!  * JOIN_WAKER - Is set to one if the JoinHandle has set a waker.
+//!  * JOIN_WAKER - Acts as an access control bit for the join handle waker. The
+//!    protocol for its usage is described below.
 //!
 //! The rest of the bits are used for the ref-count.
 //!
@@ -71,10 +72,38 @@
 //!    a lock for the stage field, and it can be accessed only by the thread
 //!    that set RUNNING to one.
 //!
-//!  * If JOIN_WAKER is zero, then the JoinHandle has exclusive access to the
-//!    join handle waker. If JOIN_WAKER and COMPLETE are both one, then the
-//!    thread that set COMPLETE to one has exclusive access to the join handle
-//!    waker.
+//!  * The waker field may be concurrently accessed by different threads: in one
+//!    thread the runtime may complete a task and *read* the waker field to
+//!    invoke the waker, and in another thread the task's JoinHandle may be
+//!    polled, and if the task hasn't yet completed, the JoinHandle may *write*
+//!    a waker to the waker field. The JOIN_WAKER bit ensures safe access by
+//!    multiple threads to the waker field using the following rules:
+//!
+//!    1. JOIN_WAKER is initialized to zero.
+//!
+//!    2. If JOIN_WAKER is zero, then the JoinHandle has exclusive (mutable)
+//!       access to the waker field.
+//!
+//!    3. If JOIN_WAKER is one, then the JoinHandle has shared (read-only)
+//!       access to the waker field.
+//!
+//!    4. If JOIN_WAKER is one and COMPLETE is one, then the runtime has shared
+//!       (read-only) access to the waker field.
+//!
+//!    5. If the JoinHandle needs to write to the waker field, then the
+//!       JoinHandle needs to (i) successfully set JOIN_WAKER to zero if it is
+//!       not already zero to gain exclusive access to the waker field per rule
+//!       2, (ii) write a waker, and (iii) successfully set JOIN_WAKER to one.
+//!
+//!    6. The JoinHandle can change JOIN_WAKER only if COMPLETE is zero (i.e.
+//!       the task hasn't yet completed).
+//!
+//!    Rule 6 implies that the steps (i) or (iii) of rule 5 may fail due to a
+//!    race. If step (i) fails, then the attempt to write a waker is aborted. If
+//!    step (iii) fails because COMPLETE is set to one by another thread after
+//!    step (i), then the waker field is cleared. Once COMPLETE is one (i.e.
+//!    task has completed), the JoinHandle will not modify JOIN_WAKER. After the
+//!    runtime sets COMPLETE to one, it invokes the waker if there is one.
 //!
 //! All other fields are immutable and can be accessed immutably without
 //! synchronization by anyone.