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. Cc: Will Deacon <will@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Boqun Feng <boqun.feng@xxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx> --- rust/kernel/block/mq/operations.rs | 7 +-- rust/kernel/block/mq/request.rs | 70 ++++++++++-------------------- rust/kernel/sync/refcount.rs | 12 +++++ 3 files changed, 38 insertions(+), 51 deletions(-) diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs index 9ba7fdfeb4b2..36ee5f96c66d 100644 --- a/rust/kernel/block/mq/operations.rs +++ b/rust/kernel/block/mq/operations.rs @@ -9,9 +9,10 @@ block::mq::request::RequestDataWrapper, block::mq::Request, error::{from_result, Result}, + 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. /// @@ -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().set(2); // 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(Refcount::new(0)) }; Ok(0) }) diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs index a0e22827f3f4..7b63c02bdce7 100644 --- a/rust/kernel/block/mq/request.rs +++ b/rust/kernel/block/mq/request.rs @@ -8,12 +8,13 @@ bindings, block::mq::Operations, error::Result, + sync::Refcount, types::{ARef, AlwaysRefCounted, Opaque}, }; use core::{ marker::PhantomData, ptr::{addr_of_mut, NonNull}, - sync::atomic::{AtomicU64, Ordering}, + sync::atomic::Ordering, }; /// A wrapper around a blk-mq `struct request`. This represents an IO request. @@ -37,6 +38,9 @@ /// We need to track C and D to ensure that it is safe to end the request and hand /// back ownership to the block layer. /// +/// Note that driver can still obtain new `ARef` even there is no `ARef`s in existence by using +/// `tag_to_rq`, hence the need to distinct 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`. @@ -91,13 +95,17 @@ pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) { /// C `struct request`. If the operation fails, `this` is returned in the /// `Err` variant. fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> { - // 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 this + .wrapper_ref() + .refcount() + .as_atomic() + .compare_exchange(2, 0, Ordering::Relaxed, Ordering::Relaxed) + .is_err() + { return Err(this); } @@ -159,13 +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: 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 } @@ -175,7 +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 Refcount { // SAFETY: Because of the safety requirements of this function, the // field projection is safe. unsafe { addr_of_mut!((*this).refcount) } @@ -191,47 +199,13 @@ unsafe impl<T: Operations> Send for Request<T> {} // mutate `self` are internally synchronized` 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) -} - -/// 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<T>` 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<T: Operations> AlwaysRefCounted for Request<T> { 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<Self>) { @@ -243,10 +217,10 @@ unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) { 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 4a5b815adc05..2cdcb20e8ee2 100644 --- a/rust/kernel/sync/refcount.rs +++ b/rust/kernel/sync/refcount.rs @@ -31,6 +31,18 @@ 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. + #[inline] + pub fn as_atomic(&self) -> &AtomicI32 { + let ptr = self.0.get() as *const AtomicI32; + // 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 are considered saturated and "bad". + unsafe { &*ptr } + } + /// Get a refcount's value. #[inline] pub fn read(&self) -> i32 { -- 2.44.1