Re: [RFC v2 00/13] LKMM *generic* atomics in Rust

[David Gow <davidgow@google.com>]

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On Fri, 1 Nov 2024 at 14:04, Boqun Feng <boqun.feng@gmail.com> wrote:
>
> Hi,
>
> This is another RFC version of LKMM atomics in Rust, you can find the
> previous versions:
>
> v1: https://lore.kernel.org/rust-for-linux/20240612223025.1158537-1-boqun.feng@gmail.com/
> wip: https://lore.kernel.org/rust-for-linux/20240322233838.868874-1-boqun.feng@gmail.com/
>
> I add a few more people Cced this time, so if you're curious about why
> we choose to implement LKMM atomics instead of using the Rust ones, you
> can find some explanation:
>
> * In my Kangrejos talk: https://lwn.net/Articles/993785/
> * In Linus' email: https://lore.kernel.org/rust-for-linux/CAHk-=whkQk=zq5XiMcaU3xj4v69+jyoP-y6Sywhq-TvxSSvfEA@mail.gmail.com/
>
> This time, I try implementing a generic atomic type `Atomic<T>`, since
> Benno and Gary suggested last time, and also Rust standard library is
> also going to that direction [1].
>
> Honestly, a generic atomic type is still not quite necessary for myself,
> but here are a few reasons that it's useful:
>
> *       It's useful for type alias, for example, if you have:
>
>         type c_long = isize;
>
>         Then `Atomic<c_clong>` and `Atomic<isize>` is the same type,
>         this may make FFI code (mapping a C type to a Rust type or vice
>         versa) more readable.
>
> *       In kernel, we sometimes switch atomic to percpu for better
>         scalabity, percpu is naturally a generic type, because it can
>         have data that is larger than machine word size. Making atomic
>         generic ease the potential switching/refactoring.
>
> *       Generic atomics provide all the functionalities that non-generic
>         atomics could have.
>
> That said, currently "generic" is limited to a few types: the type must
> be the same size as atomic_t or atomic64_t, other than basic types, only
> #[repr(<basic types>)] struct can be used in a `Atomic<T>`.
>
> Also this is not a full feature set patchset, things like different
> arithmetic operations and bit operations are still missing, they can be
> either future work or for future versions.
>
> I included an RCU pointer implementation as one example of the usage, so
> a patch from Danilo is added, but I will drop it once his patch merged.
>
> This is based on today's rust-next, and I've run all kunit tests from
> the doc test on x86, arm64 and riscv.
>
> Feedbacks and comments are welcome! Thanks.
>
> Regards,
> Boqun
>
> [1]: https://github.com/rust-lang/rust/issues/130539
>

Thanks, Boqun.

I played around a bit with porting the blk-mq atomic code to this. As
neither an expert in Rust nor an expert in atomics, this is probably
both non-idiomatic and wrong, but unlike the `core` atomics, it
provides an Atomic::<u64> on 32-bit systems, which gets UML's 32-bit
build working again.

Diff below -- I'm not likely to have much time to work on this again
soon, so feel free to pick it up/fix it if it suits.

Thanks,
-- David

---
diff --git a/rust/kernel/block/mq/operations.rs
b/rust/kernel/block/mq/operations.rs
index 9ba7fdfeb4b2..822d64230e11 100644
--- a/rust/kernel/block/mq/operations.rs
+++ b/rust/kernel/block/mq/operations.rs
@@ -11,7 +11,8 @@
     error::{from_result, Result},
     types::ARef,
 };
-use core::{marker::PhantomData, sync::atomic::AtomicU64,
sync::atomic::Ordering};
+use core::marker::PhantomData;
+use kernel::sync::atomic::{Atomic, Relaxed};

 /// Implement this trait to interface blk-mq as block devices.
 ///
@@ -77,7 +78,7 @@ impl<T: Operations> OperationsVTable<T> {
         let request = unsafe { &*(*bd).rq.cast::<Request<T>>() };

         // One refcount for the ARef, one for being in flight
-        request.wrapper_ref().refcount().store(2, Ordering::Relaxed);
+        request.wrapper_ref().refcount().store(2, Relaxed);

         // SAFETY:
         //  - We own a refcount that we took above. We pass that to `ARef`.
@@ -186,7 +187,7 @@ impl<T: Operations> OperationsVTable<T> {

             // SAFETY: The refcount field is allocated but not initialized, so
             // it is valid for writes.
-            unsafe {
RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(AtomicU64::new(0))
};
+            unsafe {
RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(Atomic::<u64>::new(0))
};

             Ok(0)
         })
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index 7943f43b9575..8d4060d65159 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -13,8 +13,8 @@
 use core::{
     marker::PhantomData,
     ptr::{addr_of_mut, NonNull},
-    sync::atomic::{AtomicU64, Ordering},
 };
+use kernel::sync::atomic::{Atomic, Relaxed};

 /// A wrapper around a blk-mq [`struct request`]. This represents an
IO request.
 ///
@@ -102,8 +102,7 @@ fn try_set_end(this: ARef<Self>) -> Result<*mut
bindings::request, ARef<Self>> {
         if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
             2,
             0,
-            Ordering::Relaxed,
-            Ordering::Relaxed,
+            Relaxed
         ) {
             return Err(this);
         }
@@ -168,13 +167,13 @@ pub(crate) struct RequestDataWrapper {
     /// - 0: The request is owned by C block layer.
     /// - 1: The request is owned by Rust abstractions but there are
no [`ARef`] references to it.
     /// - 2+: There are [`ARef`] references to the request.
-    refcount: AtomicU64,
+    refcount: Atomic::<u64>,
 }

 impl RequestDataWrapper {
     /// Return a reference to the refcount of the request that is embedding
     /// `self`.
-    pub(crate) fn refcount(&self) -> &AtomicU64 {
+    pub(crate) fn refcount(&self) -> &Atomic::<u64> {
         &self.refcount
     }

@@ -184,7 +183,7 @@ pub(crate) fn refcount(&self) -> &AtomicU64 {
     /// # Safety
     ///
     /// - `this` must point to a live allocation of at least the size
of `Self`.
-    pub(crate) unsafe fn refcount_ptr(this: *mut Self) -> *mut AtomicU64 {
+    pub(crate) unsafe fn refcount_ptr(this: *mut Self) -> *mut Atomic::<u64> {
         // SAFETY: Because of the safety requirements of this function, the
         // field projection is safe.
         unsafe { addr_of_mut!((*this).refcount) }
@@ -202,28 +201,22 @@ unsafe impl<T: Operations> Sync for Request<T> {}

 /// Store the result of `op(target.load())` in target, returning new value of
 /// target.
-fn atomic_relaxed_op_return(target: &AtomicU64, op: impl Fn(u64) ->
u64) -> u64 {
-    let old = target.fetch_update(Ordering::Relaxed,
Ordering::Relaxed, |x| Some(op(x)));
-
-    // SAFETY: Because the operation passed to `fetch_update` above always
-    // return `Some`, `old` will always be `Ok`.
-    let old = unsafe { old.unwrap_unchecked() };
-
-    op(old)
+fn atomic_relaxed_op_return(target: &Atomic::<u64>, op: impl Fn(u64)
-> u64) -> u64 {
+    let old = target.load(Relaxed);
+    let new_val = op(old);
+    target.compare_exchange(old, new_val, Relaxed);
+    old
 }

 /// Store the result of `op(target.load)` in `target` if `target.load() !=
 /// pred`, returning [`true`] if the target was updated.
-fn atomic_relaxed_op_unless(target: &AtomicU64, op: impl Fn(u64) ->
u64, pred: u64) -> bool {
-    target
-        .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| {
-            if x == pred {
-                None
-            } else {
-                Some(op(x))
-            }
-        })
-        .is_ok()
+fn atomic_relaxed_op_unless(target: &Atomic::<u64>, op: impl Fn(u64)
-> u64, pred: u64) -> bool {
+    let old = target.load(Relaxed);
+    if old == pred {
+        false
+    } else {
+        target.compare_exchange(old, op(old), Relaxed).is_ok()
+    }
 }

 // SAFETY: All instances of `Request<T>` are reference counted. This

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