[PATCH v3 2/3] rust: add a wrapper for the `nr_online_nodes` C function

[Andreas Hindborg <a.hindborg@kernel.org>]

This function returns the number of online NUMA nodes.

The wrapper is needed by the Rust null block driver.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/lib.rs  |  1 +
 rust/kernel/numa.rs | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 4d5c96ddc49c..a7b1c9cf454f 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -105,6 +105,7 @@
 #[cfg(CONFIG_NET)]
 pub mod net;
 pub mod num;
+pub mod numa;
 pub mod of;
 #[cfg(CONFIG_PM_OPP)]
 pub mod opp;
diff --git a/rust/kernel/numa.rs b/rust/kernel/numa.rs
new file mode 100644
index 000000000000..164e622d325f
--- /dev/null
+++ b/rust/kernel/numa.rs
@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! NUMA topology utilities.
+//!
+//! C header: [`include/linux/nodemask.h`](srctree/include/linux/nodemask.h)
+
+use crate::bindings;
+#[cfg(CONFIG_NUMA)]
+use crate::sync::atomic::{
+    Atomic,
+    Relaxed, //
+};
+
+/// Returns the number of online NUMA nodes.
+#[inline]
+pub fn num_online_nodes() -> u32 {
+    #[cfg(CONFIG_NUMA)]
+    {
+        // SAFETY: `nr_online_nodes` is a global variable that is valid for the lifetime of the
+        // kernel. The C side updates it with plain writes during node hotplug and reads it with
+        // plain reads. A relaxed load is the Rust equivalent of `READ_ONCE()`, which is the best
+        // we can do on the reader side.
+        unsafe { Atomic::from_ptr(&raw mut bindings::nr_online_nodes) }.load(Relaxed)
+    }
+    #[cfg(not(CONFIG_NUMA))]
+    {
+        bindings::nr_online_nodes
+    }
+}

-- 
2.54.0