linux-kernel - Re: [PATCH 1/2] rust: sync: add `CondVar::notify

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Date:   Thu, 07 Dec 2023 20:21:48 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     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>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] rust: sync: add `CondVar::notify_sync`

On 12/6/23 11:09, Alice Ryhl wrote:
> Wake up another thread synchronously.
> 
> This method behaves like `notify_one`, except that it hints to the
> scheduler that the current thread is about to go to sleep, so it should
> schedule the target thread on the same CPU.
> 
> This is used by Rust Binder as a performance optimization. When sending
> a transaction to a different process, we usually know which thread will
> handle it, so we can schedule that thread for execution next on this
> CPU for better cache locality.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/sync/condvar.rs | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
> index b679b6f6dbeb..9861c6749ad0 100644
> --- a/rust/kernel/sync/condvar.rs
> +++ b/rust/kernel/sync/condvar.rs
> @@ -155,6 +155,12 @@ fn notify(&self, count: i32, flags: u32) {
>          };
>      }
> 
> +    /// Calls the kernel function to notify one thread synchronously.
> +    pub fn notify_sync(&self) {
> +        // SAFETY: `wait_list` points to valid memory.
> +        unsafe { bindings::__wake_up_sync(self.wait_list.get(), bindings::TASK_NORMAL) };

I took a look at the C function (i.e. __wake_up_common) and there I
found this:

    lockdep_assert_held(&wq_head->lock);

So I think this function requires that the lock is held, how are you
ensuring this?

-- 
Cheers,
Benno