rust: add rcu_dereference_raw

Implement rcu_dereference_raw by calling into READ_ONCE. This function
is intended for reading pointers in an rcu_read_lock region. It
establishes a consume ordering, so it is valid to read the memory behind
the pointer.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Author: Darksonn

rust/bindings/lib.rs

@@ -50,4 +50,4 @@ mod bindings_helper {
 pub use bindings_raw::*;
 
 mod rwonce;
-pub use rwonce::{READ_ONCE, WRITE_ONCE};
+pub use rwonce::{READ_ONCE, WRITE_ONCE, rcu_dereference_raw};

rust/bindings/rwonce/mod.rs

@@ -32,3 +32,9 @@ pub unsafe fn READ_ONCE<T: Copy>(ptr: *const T) -> T {
 pub unsafe fn WRITE_ONCE<T: Copy>(ptr: *mut T, val: T) {
     unsafe { __WRITE_ONCE(ptr, val) }
 }
+
+/// Dereference the pointer for rcu.
+#[inline(always)]
+pub unsafe fn rcu_dereference_raw<T>(ptr: *const *mut T) -> *mut T {
+    unsafe { READ_ONCE(ptr) }
+}