Re: [PATCH v7 04/16] rust: add rcu abstraction

[Gary Guo <gary@garyguo.net>]

On Thu, 19 Dec 2024 18:04:06 +0100
Danilo Krummrich <dakr@kernel.org> wrote:

> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add a simple abstraction to guard critical code sections with an rcu
> read lock.
> 
> Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
> Co-developed-by: Danilo Krummrich <dakr@kernel.org>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  MAINTAINERS             |  1 +
>  rust/helpers/helpers.c  |  1 +
>  rust/helpers/rcu.c      | 13 ++++++++++++
>  rust/kernel/sync.rs     |  1 +
>  rust/kernel/sync/rcu.rs | 47 +++++++++++++++++++++++++++++++++++++++++
>  5 files changed, 63 insertions(+)
>  create mode 100644 rust/helpers/rcu.c
>  create mode 100644 rust/kernel/sync/rcu.rs

[resend to the list]

> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 3cfb68650347..0cc69e282889 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -19690,6 +19690,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev
>  F:	Documentation/RCU/
>  F:	include/linux/rcu*
>  F:	kernel/rcu/
> +F:	rust/kernel/sync/rcu.rs
>  X:	Documentation/RCU/torture.rst
>  X:	include/linux/srcu*.h
>  X:	kernel/rcu/srcu*.c
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index dcf827a61b52..060750af6524 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -20,6 +20,7 @@
>  #include "page.c"
>  #include "pid_namespace.c"
>  #include "rbtree.c"
> +#include "rcu.c"
>  #include "refcount.c"
>  #include "security.c"
>  #include "signal.c"
> diff --git a/rust/helpers/rcu.c b/rust/helpers/rcu.c
> new file mode 100644
> index 000000000000..f1cec6583513
> --- /dev/null
> +++ b/rust/helpers/rcu.c
> @@ -0,0 +1,13 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/rcupdate.h>
> +
> +void rust_helper_rcu_read_lock(void)
> +{
> +	rcu_read_lock();
> +}
> +
> +void rust_helper_rcu_read_unlock(void)
> +{
> +	rcu_read_unlock();
> +}
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 1eab7ebf25fd..0654008198b2 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -12,6 +12,7 @@
>  pub mod lock;
>  mod locked_by;
>  pub mod poll;
> +pub mod rcu;
>  
>  pub use arc::{Arc, ArcBorrow, UniqueArc};
>  pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
> diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
> new file mode 100644
> index 000000000000..b51d9150ffe2
> --- /dev/null
> +++ b/rust/kernel/sync/rcu.rs
> @@ -0,0 +1,47 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! RCU support.
> +//!
> +//! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h)
> +
> +use crate::{bindings, types::NotThreadSafe};
> +
> +/// Evidence that the RCU read side lock is held on the current thread/CPU.
> +///
> +/// The type is explicitly not `Send` because this property is per-thread/CPU.
> +///
> +/// # Invariants
> +///
> +/// The RCU read side lock is actually held while instances of this guard exist.
> +pub struct Guard(NotThreadSafe);
> +
> +impl Guard {
> +    /// Acquires the RCU read side lock and returns a guard.
> +    pub fn new() -> Self {
> +        // SAFETY: An FFI call with no additional requirements.
> +        unsafe { bindings::rcu_read_lock() };
> +        // INVARIANT: The RCU read side lock was just acquired above.
> +        Self(NotThreadSafe)
> +    }
> +
> +    /// Explicitly releases the RCU read side lock.
> +    pub fn unlock(self) {}  

I don't think there's need for this, `drop(rcu_guard)` is equally
clear.

There was a debate in Rust community about explicit lock methods, but
the conclusion was to not have it,
see https://github.com/rust-lang/rust/issues/81872.

> +}
> +
> +impl Default for Guard {
> +    fn default() -> Self {
> +        Self::new()
> +    }
> +}  

I don't think anyone would like to implicit acquire an RCU guard! I
believe you included this because clippy is yelling, but in this case
you shouldn't listen to clippy. Either suppress the warning or rename
`new` to `lock`.

> +
> +impl Drop for Guard {
> +    fn drop(&mut self) {
> +        // SAFETY: By the type invariants, the RCU read side is locked, so it is ok to unlock it.
> +        unsafe { bindings::rcu_read_unlock() };
> +    }
> +}
> +
> +/// Acquires the RCU read side lock.
> +pub fn read_lock() -> Guard {
> +    Guard::new()
> +}