From: Gary Guo Date: Fri, 5 Sep 2025 04:41:40 +0000 (-0700) Subject: rust: block: convert `block::mq` to use `Refcount` X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=a307bf1db5448eccd72a1d7857f7661c6330d5ad;p=thirdparty%2Fkernel%2Fstable.git rust: block: convert `block::mq` to use `Refcount` 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. [boqun: Adopt the LKMM atomic API] Signed-off-by: Gary Guo Signed-off-by: Boqun Feng Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Benno Lossin Reviewed-by: Elle Rhumsaa Acked-by: Andreas Hindborg Tested-by: David Gow Link: https://lore.kernel.org/r/20250723233312.3304339-5-gary@kernel.org --- diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs index c2b98f507bcbd..c0f95a9419c4e 100644 --- a/rust/kernel/block/mq/operations.rs +++ b/rust/kernel/block/mq/operations.rs @@ -10,9 +10,10 @@ use crate::{ block::mq::Request, error::{from_result, Result}, prelude::*, + sync::Refcount, types::ARef, }; -use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering}; +use core::marker::PhantomData; /// Implement this trait to interface blk-mq as block devices. /// @@ -78,7 +79,7 @@ impl OperationsVTable { let request = unsafe { &*(*bd).rq.cast::>() }; // One refcount for the ARef, one for being in flight - request.wrapper_ref().refcount().store(2, Ordering::Relaxed); + request.wrapper_ref().refcount().set(2); // SAFETY: // - We own a refcount that we took above. We pass that to `ARef`. @@ -187,7 +188,7 @@ impl OperationsVTable { // 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(Refcount::new(0)) }; Ok(0) }) diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs index fefd394f064a7..f62a376dc3139 100644 --- a/rust/kernel/block/mq/request.rs +++ b/rust/kernel/block/mq/request.rs @@ -8,13 +8,10 @@ use crate::{ bindings, block::mq::Operations, error::Result, + sync::{atomic::Relaxed, Refcount}, types::{ARef, AlwaysRefCounted, Opaque}, }; -use core::{ - marker::PhantomData, - ptr::NonNull, - sync::atomic::{AtomicU64, Ordering}, -}; +use core::{marker::PhantomData, ptr::NonNull}; /// A wrapper around a blk-mq [`struct request`]. This represents an IO request. /// @@ -37,6 +34,9 @@ use core::{ /// We need to track 3 and 4 to ensure that it is safe to end the request and hand /// back ownership to the block layer. /// +/// Note that the driver can still obtain new `ARef` even if there is no `ARef`s in existence by +/// using `tag_to_rq`, hence the need to distinguish B and C. +/// /// The states are tracked through the private `refcount` field of /// `RequestDataWrapper`. This structure lives in the private data area of the C /// [`struct request`]. @@ -98,13 +98,16 @@ impl Request { /// /// [`struct request`]: srctree/include/linux/blk-mq.h fn try_set_end(this: ARef) -> Result<*mut bindings::request, ARef> { - // We can race with `TagSet::tag_to_rq` - if let Err(_old) = this.wrapper_ref().refcount().compare_exchange( - 2, - 0, - Ordering::Relaxed, - Ordering::Relaxed, - ) { + // To hand back the ownership, we need the current refcount to be 2. + // Since we can race with `TagSet::tag_to_rq`, this needs to atomically reduce + // refcount to 0. `Refcount` does not provide a way to do this, so use the underlying + // atomics directly. + if let Err(_old) = this + .wrapper_ref() + .refcount() + .as_atomic() + .cmpxchg(2, 0, Relaxed) + { return Err(this); } @@ -173,13 +176,13 @@ 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: Refcount, } 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) -> &Refcount { &self.refcount } @@ -189,7 +192,7 @@ impl RequestDataWrapper { /// # 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 Refcount { // SAFETY: Because of the safety requirements of this function, the // field projection is safe. unsafe { &raw mut (*this).refcount } @@ -205,47 +208,13 @@ unsafe impl Send for Request {} // mutate `self` are internally synchronized` unsafe impl Sync for Request {} -/// 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) -} - -/// 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() -} - // SAFETY: All instances of `Request` are reference counted. This // implementation of `AlwaysRefCounted` ensure that increments to the ref count // keeps the object alive in memory at least until a matching reference count // decrement is executed. unsafe impl AlwaysRefCounted for Request { fn inc_ref(&self) { - let refcount = &self.wrapper_ref().refcount(); - - #[cfg_attr(not(CONFIG_DEBUG_MISC), allow(unused_variables))] - let updated = atomic_relaxed_op_unless(refcount, |x| x + 1, 0); - - #[cfg(CONFIG_DEBUG_MISC)] - if !updated { - panic!("Request refcount zero on clone") - } + self.wrapper_ref().refcount().inc(); } unsafe fn dec_ref(obj: core::ptr::NonNull) { @@ -257,10 +226,10 @@ unsafe impl AlwaysRefCounted for Request { let refcount = unsafe { &*RequestDataWrapper::refcount_ptr(wrapper_ptr) }; #[cfg_attr(not(CONFIG_DEBUG_MISC), allow(unused_variables))] - let new_refcount = atomic_relaxed_op_return(refcount, |x| x - 1); + let is_zero = refcount.dec_and_test(); #[cfg(CONFIG_DEBUG_MISC)] - if new_refcount == 0 { + if is_zero { panic!("Request reached refcount zero in Rust abstractions"); } } diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs index cc1a80ae7ae9a..19236a5bccdeb 100644 --- a/rust/kernel/sync/refcount.rs +++ b/rust/kernel/sync/refcount.rs @@ -5,6 +5,7 @@ //! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h) use crate::build_assert; +use crate::sync::atomic::Atomic; use crate::types::Opaque; /// Atomic reference counter. @@ -34,6 +35,20 @@ impl Refcount { self.0.get() } + /// Get the underlying atomic counter that backs the refcount. + /// + /// NOTE: Usage of this function is discouraged as it can circumvent the protections offered by + /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then + /// this function can be used. Otherwise consider adding a binding for the required API. + #[inline] + pub fn as_atomic(&self) -> &Atomic { + 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 `Atomic`. 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) {