linux-kernel - Re: [PATCH 7/7] rust: file: add abstraction for `poll

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Date:   Fri,  1 Dec 2023 11:47:48 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     benno.lossin@...ton.me
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.com, arve@...roid.com, bjorn3_gh@...tonmail.com,
        boqun.feng@...il.com, brauner@...nel.org, cmllamas@...gle.com,
        dan.j.williams@...el.com, dxu@...uu.xyz, gary@...yguo.net,
        gregkh@...uxfoundation.org, joel@...lfernandes.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, maco@...roid.com, ojeda@...nel.org,
        peterz@...radead.org, rust-for-linux@...r.kernel.org,
        surenb@...gle.com, tglx@...utronix.de, tkjos@...roid.com,
        viro@...iv.linux.org.uk, wedsonaf@...il.com, willy@...radead.org
Subject: Re: [PATCH 7/7] rust: file: add abstraction for `poll_table`

Benno Lossin <benno.lossin@...ton.me> writes:
>> +#[pinned_drop]
>> +impl PinnedDrop for PollCondVar {
>> +    fn drop(self: Pin<&mut Self>) {
>> +        // Clear anything registered using `register_wait`.
>> +        self.inner.notify(1, bindings::POLLHUP | bindings::POLLFREE);
> 
> Isn't notifying only a single thread problematic, since a user could
> misuse the `PollCondVar` (since all functions of `CondVar` are also
> accessible) and also `.wait()` on the condvar? When dropping a
> `PollCondVar` it might notify only the user `.wait()`, but not the
> `PollTable`. Or am I missing something?

Using POLLFREE clears everything. However, this should probably be updated to
use `wake_up_pollfree` instead.

Note that calls to `.wait()` are definitely gone by the time the destructor
runs, since such calls borrows the `PollCondVar`, preventing you from running
the destructor.

Alice