1 // SPDX-License-Identifier: GPL-2.0
2
3 //! This module provides a wrapper for the C `struct request` type.
4 //!
5 //! C header: [`include/linux/blk-mq.h`](srctree/include/linux/blk-mq.h)
6
7 use crate::{
8 bindings,
9 block::mq::Operations,
10 error::Result,
11 types::{ARef, AlwaysRefCounted, Opaque},
12 };
13 use core::{
14 marker::PhantomData,
15 ptr::{addr_of_mut, NonNull},
16 sync::atomic::{AtomicU64, Ordering},
17 };
18
19 /// A wrapper around a blk-mq `struct request`. This represents an IO request.
20 ///
21 /// # Implementation details
22 ///
23 /// There are four states for a request that the Rust bindings care about:
24 ///
25 /// A) Request is owned by block layer (refcount 0)
26 /// B) Request is owned by driver but with zero `ARef`s in existence
27 /// (refcount 1)
28 /// C) Request is owned by driver with exactly one `ARef` in existence
29 /// (refcount 2)
30 /// D) Request is owned by driver with more than one `ARef` in existence
31 /// (refcount > 2)
32 ///
33 ///
34 /// We need to track A and B to ensure we fail tag to request conversions for
35 /// requests that are not owned by the driver.
36 ///
37 /// We need to track C and D to ensure that it is safe to end the request and hand
38 /// back ownership to the block layer.
39 ///
40 /// The states are tracked through the private `refcount` field of
41 /// `RequestDataWrapper`. This structure lives in the private data area of the C
42 /// `struct request`.
43 ///
44 /// # Invariants
45 ///
46 /// * `self.0` is a valid `struct request` created by the C portion of the kernel.
47 /// * The private data area associated with this request must be an initialized
48 /// and valid `RequestDataWrapper<T>`.
49 /// * `self` is reference counted by atomic modification of
50 /// self.wrapper_ref().refcount().
51 ///
52 #[repr(transparent)]
53 pub struct Request<T: Operations>(Opaque<bindings::request>, PhantomData<T>);
54
55 impl<T: Operations> Request<T> {
56 /// Create an `ARef<Request>` from a `struct request` pointer.
57 ///
58 /// # Safety
59 ///
60 /// * The caller must own a refcount on `ptr` that is transferred to the
61 /// returned `ARef`.
62 /// * The type invariants for `Request` must hold for the pointee of `ptr`.
63 pub(crate) unsafe fn aref_from_raw(ptr: *mut bindings::request) -> ARef<Self> {
64 // INVARIANT: By the safety requirements of this function, invariants are upheld.
65 // SAFETY: By the safety requirement of this function, we own a
66 // reference count that we can pass to `ARef`.
67 unsafe { ARef::from_raw(NonNull::new_unchecked(ptr as *const Self as *mut Self)) }
68 }
69
70 /// Notify the block layer that a request is going to be processed now.
71 ///
72 /// The block layer uses this hook to do proper initializations such as
73 /// starting the timeout timer. It is a requirement that block device
74 /// drivers call this function when starting to process a request.
75 ///
76 /// # Safety
77 ///
78 /// The caller must have exclusive ownership of `self`, that is
79 /// `self.wrapper_ref().refcount() == 2`.
80 pub(crate) unsafe fn start_unchecked(this: &ARef<Self>) {
81 // SAFETY: By type invariant, `self.0` is a valid `struct request` and
82 // we have exclusive access.
83 unsafe { bindings::blk_mq_start_request(this.0.get()) };
84 }
85
86 /// Try to take exclusive ownership of `this` by dropping the refcount to 0.
87 /// This fails if `this` is not the only `ARef` pointing to the underlying
88 /// `Request`.
89 ///
90 /// If the operation is successful, `Ok` is returned with a pointer to the
91 /// C `struct request`. If the operation fails, `this` is returned in the
92 /// `Err` variant.
93 fn try_set_end(this: ARef<Self>) -> Result<*mut bindings::request, ARef<Self>> {
94 // We can race with `TagSet::tag_to_rq`
95 if let Err(_old) = this.wrapper_ref().refcount().compare_exchange(
96 2,
97 0,
98 Ordering::Relaxed,
99 Ordering::Relaxed,
100 ) {
101 return Err(this);
102 }
103
104 let request_ptr = this.0.get();
105 core::mem::forget(this);
106
107 Ok(request_ptr)
108 }
109
110 /// Notify the block layer that the request has been completed without errors.
111 ///
112 /// This function will return `Err` if `this` is not the only `ARef`
113 /// referencing the request.
114 pub fn end_ok(this: ARef<Self>) -> Result<(), ARef<Self>> {
115 let request_ptr = Self::try_set_end(this)?;
116
117 // SAFETY: By type invariant, `this.0` was a valid `struct request`. The
118 // success of the call to `try_set_end` guarantees that there are no
119 // `ARef`s pointing to this request. Therefore it is safe to hand it
120 // back to the block layer.
121 unsafe { bindings::blk_mq_end_request(request_ptr, bindings::BLK_STS_OK as _) };
122
123 Ok(())
124 }
125
126 /// Return a pointer to the `RequestDataWrapper` stored in the private area
127 /// of the request structure.
128 ///
129 /// # Safety
130 ///
131 /// - `this` must point to a valid allocation of size at least size of
132 /// `Self` plus size of `RequestDataWrapper`.
133 pub(crate) unsafe fn wrapper_ptr(this: *mut Self) -> NonNull<RequestDataWrapper> {
134 let request_ptr = this.cast::<bindings::request>();
135 // SAFETY: By safety requirements for this function, `this` is a
136 // valid allocation.
137 let wrapper_ptr =
138 unsafe { bindings::blk_mq_rq_to_pdu(request_ptr).cast::<RequestDataWrapper>() };
139 // SAFETY: By C API contract, wrapper_ptr points to a valid allocation
140 // and is not null.
141 unsafe { NonNull::new_unchecked(wrapper_ptr) }
142 }
143
144 /// Return a reference to the `RequestDataWrapper` stored in the private
145 /// area of the request structure.
146 pub(crate) fn wrapper_ref(&self) -> &RequestDataWrapper {
147 // SAFETY: By type invariant, `self.0` is a valid allocation. Further,
148 // the private data associated with this request is initialized and
149 // valid. The existence of `&self` guarantees that the private data is
150 // valid as a shared reference.
151 unsafe { Self::wrapper_ptr(self as *const Self as *mut Self).as_ref() }
152 }
153 }
154
155 /// A wrapper around data stored in the private area of the C `struct request`.
156 pub(crate) struct RequestDataWrapper {
157 /// The Rust request refcount has the following states:
158 ///
159 /// - 0: The request is owned by C block layer.
160 /// - 1: The request is owned by Rust abstractions but there are no ARef references to it.
161 /// - 2+: There are `ARef` references to the request.
162 refcount: AtomicU64,
163 }
164
165 impl RequestDataWrapper {
166 /// Return a reference to the refcount of the request that is embedding
167 /// `self`.
168 pub(crate) fn refcount(&self) -> &AtomicU64 {
169 &self.refcount
170 }
171
172 /// Return a pointer to the refcount of the request that is embedding the
173 /// pointee of `this`.
174 ///
175 /// # Safety
176 ///
177 /// - `this` must point to a live allocation of at least the size of `Self`.
178 pub(crate) unsafe fn refcount_ptr(this: *mut Self) -> *mut AtomicU64 {
179 // SAFETY: Because of the safety requirements of this function, the
180 // field projection is safe.
181 unsafe { addr_of_mut!((*this).refcount) }
182 }
183 }
184
185 // SAFETY: Exclusive access is thread-safe for `Request`. `Request` has no `&mut
186 // self` methods and `&self` methods that mutate `self` are internally
187 // synchronized.
188 unsafe impl<T: Operations> Send for Request<T> {}
189
190 // SAFETY: Shared access is thread-safe for `Request`. `&self` methods that
191 // mutate `self` are internally synchronized`
192 unsafe impl<T: Operations> Sync for Request<T> {}
193
194 /// Store the result of `op(target.load())` in target, returning new value of
195 /// target.
196 fn atomic_relaxed_op_return(target: &AtomicU64, op: impl Fn(u64) -> u64) -> u64 {
197 let old = target.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| Some(op(x)));
198
199 // SAFETY: Because the operation passed to `fetch_update` above always
200 // return `Some`, `old` will always be `Ok`.
201 let old = unsafe { old.unwrap_unchecked() };
202
203 op(old)
204 }
205
206 /// Store the result of `op(target.load)` in `target` if `target.load() !=
207 /// pred`, returning true if the target was updated.
208 fn atomic_relaxed_op_unless(target: &AtomicU64, op: impl Fn(u64) -> u64, pred: u64) -> bool {
209 target
210 .fetch_update(Ordering::Relaxed, Ordering::Relaxed, |x| {
211 if x == pred {
212 None
213 } else {
214 Some(op(x))
215 }
216 })
217 .is_ok()
218 }
219
220 // SAFETY: All instances of `Request<T>` are reference counted. This
221 // implementation of `AlwaysRefCounted` ensure that increments to the ref count
222 // keeps the object alive in memory at least until a matching reference count
223 // decrement is executed.
224 unsafe impl<T: Operations> AlwaysRefCounted for Request<T> {
225 fn inc_ref(&self) {
226 let refcount = &self.wrapper_ref().refcount();
227
228 #[cfg_attr(not(CONFIG_DEBUG_MISC), allow(unused_variables))]
229 let updated = atomic_relaxed_op_unless(refcount, |x| x + 1, 0);
230
231 #[cfg(CONFIG_DEBUG_MISC)]
232 if !updated {
233 panic!("Request refcount zero on clone")
234 }
235 }
236
237 unsafe fn dec_ref(obj: core::ptr::NonNull<Self>) {
238 // SAFETY: The type invariants of `ARef` guarantee that `obj` is valid
239 // for read.
240 let wrapper_ptr = unsafe { Self::wrapper_ptr(obj.as_ptr()).as_ptr() };
241 // SAFETY: The type invariant of `Request` guarantees that the private
242 // data area is initialized and valid.
243 let refcount = unsafe { &*RequestDataWrapper::refcount_ptr(wrapper_ptr) };
244
245 #[cfg_attr(not(CONFIG_DEBUG_MISC), allow(unused_variables))]
246 let new_refcount = atomic_relaxed_op_return(refcount, |x| x - 1);
247
248 #[cfg(CONFIG_DEBUG_MISC)]
249 if new_refcount == 0 {
250 panic!("Request reached refcount zero in Rust abstractions");
251 }
252 }
253 }
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