Re: [PATCH v5 04/16] rust: add rcu abstraction

[Boqun Feng <boqun.feng@gmail.com>]

Hi Danilo,

This looks good so far, could you please Cc RCU maintainers for the next
version (if there is one).

A few nits below, with or without, feel free to add:

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

On Tue, Dec 10, 2024 at 11:46:31PM +0100, Danilo Krummrich wrote:
> From: Wedson Almeida Filho <wedsonaf@gmail.com>
> 
> Add a simple abstraction to guard critical code sections with an rcu
> read lock.
> 
> 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>
> ---
>  rust/helpers/helpers.c  |  1 +
>  rust/helpers/rcu.c      | 13 ++++++++++++
>  rust/kernel/sync.rs     |  1 +
>  rust/kernel/sync/rcu.rs | 47 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 62 insertions(+)
>  create mode 100644 rust/helpers/rcu.c
>  create mode 100644 rust/kernel/sync/rcu.rs
> 
> 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..3beef70d5f34
> --- /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) {}
> +}
> +
> +impl Default for Guard {
> +    fn default() -> Self {
> +        Self::new()
> +    }
> +}
> +
> +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.

s/rcu/RCU

when referring the RCU mechanism or the subsystem, it makes sense to use
the capitalized abbreviation.

Regards,
Boqun

> +        unsafe { bindings::rcu_read_unlock() };
> +    }
> +}
> +
> +/// Acquires the RCU read side lock.
> +pub fn read_lock() -> Guard {
> +    Guard::new()
> +}
> -- 
> 2.47.0
>