linux-kernel - [RFC v2 11/13] rust: sync: Add memory barriers

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Message-ID: <20241101060237.1185533-12-boqun.feng@gmail.com>
Date: Thu, 31 Oct 2024 23:02:34 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: rust-for-linux@...r.kernel.org,
	rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev,
	lkmm@...ts.linux.dev
Cc: Miguel Ojeda <ojeda@...nel.org>,	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
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	linux-riscv@...ts.infradead.org
Subject: [RFC v2 11/13] rust: sync: Add memory barriers

Memory barriers are building blocks for concurrent code, hence provide
a minimal set of them.

The compiler barrier, barrier(), is implemented in inline asm instead of
using core::sync::atomic::compiler_fence() because memory models are
different: kernel's atomics are implemented in inline asm therefore the
compiler barrier should be implemented in inline asm as well.

Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 rust/helpers/helpers.c      |  1 +
 rust/kernel/sync.rs         |  1 +
 rust/kernel/sync/barrier.rs | 67 +++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)
 create mode 100644 rust/kernel/sync/barrier.rs

diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index ab5a3f1be241..f4a94833b29d 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -8,6 +8,7 @@
  */
 
 #include "atomic.c"
+#include "barrier.c"
 #include "blk.c"
 #include "bug.c"
 #include "build_assert.c"
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 66ac3752ca71..0d0b19441ae8 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -9,6 +9,7 @@
 
 mod arc;
 pub mod atomic;
+pub mod barrier;
 mod condvar;
 pub mod lock;
 mod locked_by;
diff --git a/rust/kernel/sync/barrier.rs b/rust/kernel/sync/barrier.rs
new file mode 100644
index 000000000000..277aa09747bf
--- /dev/null
+++ b/rust/kernel/sync/barrier.rs
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Memory barriers.
+//!
+//! These primitives have the same semantics as their C counterparts: and the precise definitions of
+//! semantics can be found at [`LKMM`].
+//!
+//! [`LKMM`]: srctree/tools/memory-mode/
+
+/// A compiler barrier.
+///
+/// An explicic compiler barrier function that prevents the compiler from moving the memory
+/// accesses either side of it to the other side.
+pub fn barrier() {
+    // By default, Rust inline asms are treated as being able to access any memory or flags, hence
+    // it suffices as a compiler barrier.
+    //
+    // SAFETY: An empty asm block should be safe.
+    unsafe {
+        core::arch::asm!("");
+    }
+}
+
+/// A full memory barrier.
+///
+/// A barrier function that prevents both the compiler and the CPU from moving the memory accesses
+/// either side of it to the other side.
+pub fn smp_mb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_mb()` is safe to call.
+        unsafe {
+            bindings::smp_mb();
+        }
+    } else {
+        barrier();
+    }
+}
+
+/// A write-write memory barrier.
+///
+/// A barrier function that prevents both the compiler and the CPU from moving the memory write
+/// accesses either side of it to the other side.
+pub fn smp_wmb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_wmb()` is safe to call.
+        unsafe {
+            bindings::smp_wmb();
+        }
+    } else {
+        barrier();
+    }
+}
+
+/// A read-read memory barrier.
+///
+/// A barrier function that prevents both the compiler and the CPU from moving the memory read
+/// accesses either side of it to the other side.
+pub fn smp_rmb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_rmb()` is safe to call.
+        unsafe {
+            bindings::smp_rmb();
+        }
+    } else {
+        barrier();
+    }
+}
-- 
2.45.2