LKMM *generic* atomics in Rust

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

Posted by Boqun Feng 3 weeks, 2 days ago

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@gmail.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

Re: [RFC v2 11/13] rust: sync: Add memory barriers

Posted by David Gow 3 weeks, 2 days ago

On Fri, 1 Nov 2024 at 14:07, Boqun Feng <boqun.feng@gmail.com> wrote:
>
> 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@gmail.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"

It looks like "barrier.c" is missing, so this isn't compiling for me.
I assume it was meant to be added in this patch...?

Thanks,
-- David

Re: [RFC v2 11/13] rust: sync: Add memory barriers

Posted by Boqun Feng 3 weeks, 2 days ago

On Fri, Nov 01, 2024 at 02:55:23PM +0800, David Gow wrote:
> On Fri, 1 Nov 2024 at 14:07, Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > 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@gmail.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"
> 
> It looks like "barrier.c" is missing, so this isn't compiling for me.
> I assume it was meant to be added in this patch...?
> 

Yes, I just send an updated one.

Glad being "checking missing files" buddies with you ;-) Thanks!

Regards,
Boqun

> Thanks,
> -- David

[RFC v2.1 11/13] rust: sync: Add memory barriers

Posted by Boqun Feng 3 weeks, 2 days ago

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@gmail.com>
---
v2 -> v2.1: Add the missing barrier.c

 rust/helpers/barrier.c      | 18 ++++++++++
 rust/helpers/helpers.c      |  1 +
 rust/kernel/sync.rs         |  1 +
 rust/kernel/sync/barrier.rs | 67 +++++++++++++++++++++++++++++++++++++
 4 files changed, 87 insertions(+)
 create mode 100644 rust/helpers/barrier.c
 create mode 100644 rust/kernel/sync/barrier.rs

diff --git a/rust/helpers/barrier.c b/rust/helpers/barrier.c
new file mode 100644
index 000000000000..cdf28ce8e511
--- /dev/null
+++ b/rust/helpers/barrier.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <asm/barrier.h>
+
+void rust_helper_smp_mb(void)
+{
+	smp_mb();
+}
+
+void rust_helper_smp_wmb(void)
+{
+	smp_wmb();
+}
+
+void rust_helper_smp_rmb(void)
+{
+	smp_rmb();
+}
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