Re: [PATCH v7 3/5] rust: sync: Add abstraction for rcu_barrier()

On Wed, Jul 29, 2026 at 11:45:40AM +0200, Philipp Stanner wrote:
> rcu_barrier() is a frequently used C function which is always safe to be
> called.

Just checking...  Here "always safe" means only from task level, with BH,
preemption, and interrupts all enabled, correct?

In contrast, if you do this:

	preempt_disable();
	rcu_barrier();
	preempt_enable();

the results won't be safe.

							Thanx, Paul

> Add a safe abstraction for rcu_barrier().
> 
> Signed-off-by: Philipp Stanner <[email protected]>
> Tested-by: Daniel Almeida <[email protected]>
> ---
>  rust/kernel/sync/rcu.rs | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs
> index a32bef6e490b..7031ca5d2473 100644
> --- a/rust/kernel/sync/rcu.rs
> +++ b/rust/kernel/sync/rcu.rs
> @@ -50,3 +50,23 @@ fn drop(&mut self) {
>  pub fn read_lock() -> Guard {
>      Guard::new()
>  }
> +
> +/// Wait until all in-flight call_rcu() callbacks complete.
> +///
> +/// Note that this primitive does not necessarily wait for an RCU grace period
> +/// to complete.  For example, if there are no RCU callbacks queued anywhere
> +/// in the system, then rcu_barrier() is within its rights to return
> +/// immediately, without waiting for anything, much less an RCU grace period.
> +/// In fact, rcu_barrier() will normally not result in any RCU grace periods
> +/// beyond those that were already destined to be executed.
> +///
> +/// In kernels built with CONFIG_RCU_LAZY=y, this function also hurries all
> +/// pending lazy RCU callbacks.
> +///
> +/// Note that this is one of the RCU primitives which must not be called in
> +/// atomic context.
> +#[inline]
> +pub fn rcu_barrier() {
> +    // SAFETY: `rcu_barrier()` is always safe to be called. It just might wait for a grace period.
> +    unsafe { bindings::rcu_barrier() };
> +}
> -- 
> 2.55.0
>