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

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Dec 24, 2024 at 08:54:50PM +0000, Gary Guo wrote:
> 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.

I don't mind one or the other, feel free to send a patch to remove it. :)

> 
> 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`.

I picked up this patch from Wedson, so I can't tell for sure. I don't see
any other reason for this though, so we could remove it.

> 
> > +
> > +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()
> > +}  
>