Re: [PATCH v5 4/5] rust: block: convert `block::mq` to use `Refcount`
From: "Benno Lossin" <lossin@kernel.org>
Date: 2025-08-12 08:17:50
Also in:
lkml, rust-for-linux
On Thu Jul 24, 2025 at 1:32 AM CEST, Gary Guo wrote:
From: Gary Guo <gary@garyguo.net> Currently there's a custom reference counting in `block::mq`, which uses `AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit architectures. We cannot just change it to use 32-bit atomics, because doing so will make it vulnerable to refcount overflow. So switch it to use the kernel refcount `kernel::sync::Refcount` instead. There is an operation needed by `block::mq`, atomically decreasing refcount from 2 to 0, which is not available through refcount.h, so I exposed `Refcount::as_atomic` which allows accessing the refcount directly. Tested-by: David Gow <redacted> Acked-by: Andreas Hindborg <a.hindborg@kernel.org> Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
--- rust/kernel/block/mq/operations.rs | 7 ++-- rust/kernel/block/mq/request.rs | 63 ++++++++---------------------- rust/kernel/sync/refcount.rs | 14 +++++++ 3 files changed, 34 insertions(+), 50 deletions(-)
quoted hunk ↗ jump to hunk
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs index 3ff4585326b41..a9b24c6b2f8a7 100644 --- a/rust/kernel/sync/refcount.rs +++ b/rust/kernel/sync/refcount.rs@@ -4,6 +4,8 @@ //! //! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h) +use core::sync::atomic::AtomicI32; + use crate::build_assert; use crate::types::Opaque;@@ -34,6 +36,18 @@ fn as_ptr(&self) -> *mut bindings::refcount_t { self.0.get() } + /// Get the underlying atomic counter that backs the refcount. + /// + /// NOTE: This will be changed to LKMM atomic in the future.
Can we discourage using this function a bit more in the docs? At least point people to try other ways before reaching for this, since it allows overflowing & doesn't warn on saturate etc. --- Cheers, Benno
+ #[inline]
+ pub fn as_atomic(&self) -> &AtomicI32 {
+ let ptr = self.0.get().cast();
+ // SAFETY: `refcount_t` is a transparent wrapper of `atomic_t`, which is an atomic 32-bit
+ // integer that is layout-wise compatible with `AtomicI32`. All values are valid for
+ // `refcount_t`, despite some of the values being considered saturated and "bad".
+ unsafe { &*ptr }
+ }
+
/// Set a refcount's value.
#[inline]
pub fn set(&self, value: i32) {