linux-kernel - Re: [PATCH v4 9/9] rust: file: add abstraction for `poll

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Date: Wed, 7 Feb 2024 21:32:58 -0300
From: Martin Rodriguez Reboredo <yakoyoku@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng
 <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Arve Hjønnevåg <arve@...roid.com>,
 Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
 Joel Fernandes <joel@...lfernandes.org>, Carlos Llamas
 <cmllamas@...gle.com>, Suren Baghdasaryan <surenb@...gle.com>,
 Dan Williams <dan.j.williams@...el.com>, Kees Cook <keescook@...omium.org>,
 Matthew Wilcox <willy@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Cc: Daniel Xu <dxu@...uu.xyz>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v4 9/9] rust: file: add abstraction for `poll_table`

On 2/2/24 07:55, Alice Ryhl wrote:
> The existing `CondVar` abstraction is a wrapper around
> `wait_queue_head`, but it does not support all use-cases of the C
> `wait_queue_head` type. To be specific, a `CondVar` cannot be registered
> with a `struct poll_table`. This limitation has the advantage that you
> do not need to call `synchronize_rcu` when destroying a `CondVar`.
> 
> However, we need the ability to register a `poll_table` with a
> `wait_queue_head` in Rust Binder. To enable this, introduce a type
> called `PollCondVar`, which is like `CondVar` except that you can
> register a `poll_table`. We also introduce `PollTable`, which is a safe
> wrapper around `poll_table` that is intended to be used with
> `PollCondVar`.
> 
> The destructor of `PollCondVar` unconditionally calls `synchronize_rcu`
> to ensure that the removal of epoll waiters has fully completed before
> the `wait_queue_head` is destroyed.
> 
> That said, `synchronize_rcu` is rather expensive and is not needed in
> all cases: If we have never registered a `poll_table` with the
> `wait_queue_head`, then we don't need to call `synchronize_rcu`. (And
> this is a common case in Binder - not all processes use Binder with
> epoll.) The current implementation does not account for this, but if we
> find that it is necessary to improve this, a future patch could store a
> boolean next to the `wait_queue_head` to keep track of whether a
> `poll_table` has ever been registered.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>