// 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");
}
}
}
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