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Diffstat (limited to 'rust/kernel/block/mq/request.rs')
| -rw-r--r-- | rust/kernel/block/mq/request.rs | 253 |
1 files changed, 253 insertions, 0 deletions
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs new file mode 100644 index 000000000000..a0e22827f3f4 --- /dev/null +++ b/rust/kernel/block/mq/request.rs @@ -0,0 +1,253 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! This module provides a wrapper for the C `struct request` type. +//! +//! C header: [`include/linux/blk-mq.h`](srctree/include/linux/blk-mq.h) + +use crate::{ + bindings, + block::mq::Operations, + error::Result, + types::{ARef, AlwaysRefCounted, Opaque}, +}; +use core::{ + marker::PhantomData, + ptr::{addr_of_mut, NonNull}, + sync::atomic::{AtomicU64, Ordering}, +}; + +/// A wrapper around a blk-mq `struct request`. This represents an IO request. +/// +/// # Implementation details +/// +/// There are four states for a request that the Rust bindings care about: +/// +/// A) Request is owned by block layer (refcount 0) +/// B) Request is owned by driver but with zero `ARef`s in existence +/// (refcount 1) +/// C) Request is owned by driver with exactly one `ARef` in existence +/// (refcount 2) +/// D) Request is owned by driver with more than one `ARef` in existence +/// (refcount > 2) +/// +/// +/// We need to track A and B to ensure we fail tag to request conversions for +/// requests that are not owned by the driver. +/// +/// 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. +/// +/// The states are tracked through the private `refcount` field of +/// `RequestDataWrapper`. This structure lives in the private data area of the C +/// `struct request`. +/// +/// # Invariants +/// +/// * `self.0` is a valid `struct request` created by the C portion of the kernel. +/// * The private data area associated with this request must be an initialized +/// and valid `RequestDataWrapper<T>`. +/// * `self` is reference counted by atomic modification of +/// self.wrapper_ref().refcount(). +/// +#[repr(transparent)] +pub struct Request<T: Operations>(Opaque<bindings::request>, PhantomData<T>); + +impl<T: Operations> Request<T> { + /// Create an `ARef<Request>` from a `struct request` pointer. + /// + /// # Safety + /// + /// * The caller must own a refcount on `ptr` that is transferred to the + /// returned `ARef`. + /// * The type invariants for `Request` must hold for the pointee of `ptr`. + pub(crate) unsafe fn aref_from_raw(ptr: *mut bindings::request) -> ARef<Self> { + // INVARIANT: By the safety requirements of this function, invariants are upheld. + // SAFETY: By the safety requirement of this function, we own a + // reference count that we can pass to `ARef`. + unsafe { ARef::from_raw(NonNull::new_unchecked(ptr as *const Self as *mut Self)) } + } + + /// Notify the block layer that a request is going to be processed now. + /// + /// The block layer uses this hook to do proper initializations such as + /// starting the timeout timer. It is a requirement that block device + /// drivers call this function when starting to process a request. + /// + /// # Safety + /// + /// The caller must have exclusive ownership of `self`, that is + /// `self.wrapper_ref().refcount() == 2`. + pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) { + // SAFETY: By type invariant, `self.0` is a valid `struct request` and + // we have exclusive access. + unsafe { bindings::blk_mq_start_request(this.0.get()) }; + } + + /// Try to take exclusive ownership of `this` by dropping the refcount to 0. + /// This fails if `this` is not the only `ARef` pointing to the underlying + /// `Request`. + /// + /// If the operation is successful, `Ok` is returned with a pointer to the + /// 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, + ) { + return Err(this); + } + + let request_ptr = this.0.get(); + core::mem::forget(this); + + Ok(request_ptr) + } + + /// Notify the block layer that the request has been completed without errors. + /// + /// This function will return `Err` if `this` is not the only `ARef` + /// referencing the request. + pub fn end_ok(this: ARef<Self>) -> Result<(), ARef<Self>> { + let request_ptr = Self::try_set_end(this)?; + + // SAFETY: By type invariant, `this.0` was a valid `struct request`. The + // success of the call to `try_set_end` guarantees that there are no + // `ARef`s pointing to this request. Therefore it is safe to hand it + // back to the block layer. + unsafe { bindings::blk_mq_end_request(request_ptr, bindings::BLK_STS_OK as _) }; + + Ok(()) + } + + /// Return a pointer to the `RequestDataWrapper` stored in the private area + /// of the request structure. + /// + /// # Safety + /// + /// - `this` must point to a valid allocation of size at least size of + /// `Self` plus size of `RequestDataWrapper`. + pub(crate) unsafe fn wrapper_ptr(this: *mut Self) -> NonNull<RequestDataWrapper> { + let request_ptr = this.cast::<bindings::request>(); + // SAFETY: By safety requirements for this function, `this` is a + // valid allocation. + let wrapper_ptr = + unsafe { bindings::blk_mq_rq_to_pdu(request_ptr).cast::<RequestDataWrapper>() }; + // SAFETY: By C API contract, wrapper_ptr points to a valid allocation + // and is not null. + unsafe { NonNull::new_unchecked(wrapper_ptr) } + } + + /// Return a reference to the `RequestDataWrapper` stored in the private + /// area of the request structure. + pub(crate) fn wrapper_ref(&self) -> &RequestDataWrapper { + // SAFETY: By type invariant, `self.0` is a valid allocation. Further, + // the private data associated with this request is initialized and + // valid. The existence of `&self` guarantees that the private data is + // valid as a shared reference. + unsafe { Self::wrapper_ptr(self as *const Self as *mut Self).as_ref() } + } +} + +/// A wrapper around data stored in the private area of the C `struct request`. +pub(crate) struct RequestDataWrapper { + /// The Rust request refcount has the following states: + /// + /// - 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, +} + +impl RequestDataWrapper { + /// Return a reference to the refcount of the request that is embedding + /// `self`. + pub(crate) fn refcount(&self) -> &AtomicU64 { + &self.refcount + } + + /// Return a pointer to the refcount of the request that is embedding the + /// pointee of `this`. + /// + /// # 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 { + // SAFETY: Because of the safety requirements of this function, the + // field projection is safe. + unsafe { addr_of_mut!((*this).refcount) } + } +} + +// SAFETY: Exclusive access is thread-safe for `Request`. `Request` has no `&mut +// self` methods and `&self` methods that mutate `self` are internally +// synchronized. +unsafe impl<T: Operations> Send for Request<T> {} + +// SAFETY: Shared access is thread-safe for `Request`. `&self` methods that +// 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") + } + } + + unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) { + // SAFETY: The type invariants of `ARef` guarantee that `obj` is valid + // for read. + let wrapper_ptr = unsafe { Self::wrapper_ptr(obj.as_ptr()).as_ptr() }; + // SAFETY: The type invariant of `Request` guarantees that the private + // data area is initialized and valid. + 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); + + #[cfg(CONFIG_DEBUG_MISC)] + if new_refcount == 0 { + panic!("Request reached refcount zero in Rust abstractions"); + } + } +} |