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Subject: Re: [PATCH v2 7/7] rust: file: add abstraction for `poll_table`
From:   Alice Ryhl <aliceryhl@google.com>
To:     benno.lossin@proton.me
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Benno Lossin <benno.lossin@proton.me> writes:
>> and here we can said,
>> 
>> "per type invariant, `qproc` cannot publish `cv.wait_list` without
>> proper RCU protection, so it's safe to use `cv.wait_list` here, and with
>> the synchronize_rcu() in PollCondVar::drop(), free of the wait_list will
>> be delayed until all usages are done."
> 
> I think I am missing how the call to `__wake_up_pollfree` ensures that
> nobody uses the `PollCondVar` any longer. How is it removed from the
> table?

The __wake_up_pollfree function clears the queue. Here is its
documentation:

/**
 * wake_up_pollfree - signal that a polled waitqueue is going away
 * @wq_head: the wait queue head
 *
 * In the very rare cases where a ->poll() implementation uses a waitqueue whose
 * lifetime is tied to a task rather than to the 'struct file' being polled,
 * this function must be called before the waitqueue is freed so that
 * non-blocking polls (e.g. epoll) are notified that the queue is going away.
 *
 * The caller must also RCU-delay the freeing of the wait_queue_head, e.g. via
 * an explicit synchronize_rcu() or call_rcu(), or via SLAB_TYPESAFE_BY_RCU.
 */

The only way for another thread to touch the queue after it has been
cleared is if they are concurrently removing themselves from the queue
under RCU. Because of that, we have to wait for an RCU grace period
after the call to __wake_up_pollfree to ensure that any such concurrent
users have gone away.

Alice