Unknown which places in tokio::sync cause deadlock

[amab8901]

This is a branch from #5429.

Description:
There are some types in tokio::sync which keep a shared state behind a mutex and expose methods to modify it through a shared reference. Two instances were uncovered. One was solved and merged, while the other one is waiting to avoid merge conflict. Now we need to identify all other instances in tokio::sync where this bug occurs. I found that the key thing that is held in common among lines that produce this deadlock is that they have a statement containing the waiter keyword. Applying appropriate changes to each of the below lines might result in a guarantee of this deadlock bug never occurring again in tokio::sync.

Task: go through every type in tokio::sync and check whether they have this bug.

Lines containing waker in statement:
Lines containing waker in statement:

• mpsc/chan.rs
  • 327 (initialize)
• mpsc/unbounded.rs
  • no statements
• task/atomic_waker.rs
  • 195 (initialize)
  • 200 (initialize)
  • 204 (risky assignment)
  • 237 (initialize)
  • 246 (old waker saved)
  • 251 (condition)
  • 262 (condition)
  • 304 (condition)
  • 320 (initialize)
• barrier.rs
  • no statements
• batch_semaphore.rs
  • 232 (initialize)
  • 233 (condition)
  • 285 (initialize)
  • 306 (condition)
  • 452 (reformulation)
  • 459 (old waker saved)
• broadcast.rs
  • 772 (initialize)
  • 903 (condition)
  • 915 (risky assignment)
  • 1169 (condition)
• notify.rs
  • 464 (condition)
  • 503 (initialize)
  • 533 (condition)
  • 603 (initialize)
  • 771 (risky assignment)
  • 835 (risky assignment)
  • 868 (condition)
  • 874 (risky assignment)
  • 939 (condition)
• oneshot.rs
  • 412 (initialize)
  • 419 (initialize)
  • 427 (risky write)

[amab8901]

so these are the lines where I think the deadlock bug is most likely to appear:

• high-risk shortlist:
  • task/atomic_waker.rs
    • 204 (risky assignment)
  • broadcast.rs
    • 915 (risky assignment)
  • notify.rs
    • 771 (risky assignment)
    • 835 (risky assignment)
    • 874 (risky assignment)
  • oneshot.rs
    • 427 (risky write)