linux-kernel - Re: [PATCH v2 3/4] rust: sync: add `CondVar::wait

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Message-ID: <ZYC2W5Rzdrsr2KKv@boqun-archlinux>
Date: Mon, 18 Dec 2023 13:15:07 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...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>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, Tiago Lam <tiagolam@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] rust: sync: add `CondVar::wait_timeout`

On Sat, Dec 16, 2023 at 03:31:41PM +0000, Alice Ryhl wrote:
> Sleep on a condition variable with a timeout.
> 
> This is used by Rust Binder for process freezing. There, we want to
> sleep until the freeze operation completes, but we want to be able to
> abort the process freezing if it doesn't complete within some timeout.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> ---
>  rust/kernel/sync/condvar.rs | 59 ++++++++++++++++++++++++++++++++++++++++-----
>  rust/kernel/sync/lock.rs    |  4 +--
>  2 files changed, 55 insertions(+), 8 deletions(-)
> 
> diff --git a/rust/kernel/sync/condvar.rs b/rust/kernel/sync/condvar.rs
> index 9331eb606738..0176cdfced6c 100644
> --- a/rust/kernel/sync/condvar.rs
> +++ b/rust/kernel/sync/condvar.rs
> @@ -6,7 +6,8 @@
>  //! variable.
>  
>  use super::{lock::Backend, lock::Guard, LockClassKey};
> -use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque};
> +use crate::{bindings, init::PinInit, pin_init, str::CStr, time::Jiffies, types::Opaque};
> +use core::ffi::c_long;
>  use core::marker::PhantomPinned;
>  use macros::pin_data;
>  
> @@ -18,6 +19,8 @@ macro_rules! new_condvar {
>      };
>  }
>  
> +const MAX_SCHEDULE_TIMEOUT: c_long = c_long::MAX;
> +

I'd like to put this in rust/kernel/time.rs or rust/kernel/task.rs, but
it's not a blocker.

>  /// A conditional variable.
>  ///
>  /// Exposes the kernel's [`struct wait_queue_head`] as a condition variable. It allows the caller to
> @@ -102,7 +105,12 @@ pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self
>          })
>      }
>  
> -    fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guard<'_, T, B>) {
> +    fn wait_internal<T: ?Sized, B: Backend>(
> +        &self,
> +        wait_state: u32,
> +        guard: &mut Guard<'_, T, B>,
> +        timeout: c_long,

Nit: maybe `timeout_in_jiffies` instead of `timeout`? Or we have another
data type:

	pub type DeltaJiffies = c_long;

or 

	pub type JiffyDelta = c_long;

because a "c_long timeout" really hurts the readability.

Regards,
Boqun

> +    ) -> c_long {
>          let wait = Opaque::<bindings::wait_queue_entry>::uninit();
>  
>          // SAFETY: `wait` points to valid memory.
> @@ -113,11 +121,13 @@ fn wait_internal<T: ?Sized, B: Backend>(&self, wait_state: u32, guard: &mut Guar
>              bindings::prepare_to_wait_exclusive(self.wait_list.get(), wait.get(), wait_state as _)
>          };
>  
[...]