kernel/sync/arc.rs
1// SPDX-License-Identifier: GPL-2.0
2
3//! A reference-counted pointer.
4//!
5//! This module implements a way for users to create reference-counted objects and pointers to
6//! them. Such a pointer automatically increments and decrements the count, and drops the
7//! underlying object when it reaches zero. It is also safe to use concurrently from multiple
8//! threads.
9//!
10//! It is different from the standard library's [`Arc`] in a few ways:
11//! 1. It is backed by the kernel's `refcount_t` type.
12//! 2. It does not support weak references, which allows it to be half the size.
13//! 3. It saturates the reference count instead of aborting when it goes over a threshold.
14//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
15//! 5. The object in [`Arc`] is pinned implicitly.
16//!
17//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
18
19use crate::{
20 alloc::{AllocError, Flags, KBox},
21 bindings,
22 init::{self, InPlaceInit, Init, PinInit},
23 try_init,
24 types::{ForeignOwnable, Opaque},
25};
26use core::{
27 alloc::Layout,
28 fmt,
29 marker::PhantomData,
30 mem::{ManuallyDrop, MaybeUninit},
31 ops::{Deref, DerefMut},
32 pin::Pin,
33 ptr::NonNull,
34};
35use macros::pin_data;
36
37mod std_vendor;
38
39/// A reference-counted pointer to an instance of `T`.
40///
41/// The reference count is incremented when new instances of [`Arc`] are created, and decremented
42/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped.
43///
44/// # Invariants
45///
46/// The reference count on an instance of [`Arc`] is always non-zero.
47/// The object pointed to by [`Arc`] is always pinned.
48///
49/// # Examples
50///
51/// ```
52/// use kernel::sync::Arc;
53///
54/// struct Example {
55/// a: u32,
56/// b: u32,
57/// }
58///
59/// // Create a refcounted instance of `Example`.
60/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
61///
62/// // Get a new pointer to `obj` and increment the refcount.
63/// let cloned = obj.clone();
64///
65/// // Assert that both `obj` and `cloned` point to the same underlying object.
66/// assert!(core::ptr::eq(&*obj, &*cloned));
67///
68/// // Destroy `obj` and decrement its refcount.
69/// drop(obj);
70///
71/// // Check that the values are still accessible through `cloned`.
72/// assert_eq!(cloned.a, 10);
73/// assert_eq!(cloned.b, 20);
74///
75/// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
76/// # Ok::<(), Error>(())
77/// ```
78///
79/// Using `Arc<T>` as the type of `self`:
80///
81/// ```
82/// use kernel::sync::Arc;
83///
84/// struct Example {
85/// a: u32,
86/// b: u32,
87/// }
88///
89/// impl Example {
90/// fn take_over(self: Arc<Self>) {
91/// // ...
92/// }
93///
94/// fn use_reference(self: &Arc<Self>) {
95/// // ...
96/// }
97/// }
98///
99/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
100/// obj.use_reference();
101/// obj.take_over();
102/// # Ok::<(), Error>(())
103/// ```
104///
105/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
106///
107/// ```
108/// use kernel::sync::{Arc, ArcBorrow};
109///
110/// trait MyTrait {
111/// // Trait has a function whose `self` type is `Arc<Self>`.
112/// fn example1(self: Arc<Self>) {}
113///
114/// // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
115/// fn example2(self: ArcBorrow<'_, Self>) {}
116/// }
117///
118/// struct Example;
119/// impl MyTrait for Example {}
120///
121/// // `obj` has type `Arc<Example>`.
122/// let obj: Arc<Example> = Arc::new(Example, GFP_KERNEL)?;
123///
124/// // `coerced` has type `Arc<dyn MyTrait>`.
125/// let coerced: Arc<dyn MyTrait> = obj;
126/// # Ok::<(), Error>(())
127/// ```
128#[repr(transparent)]
129#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
130pub struct Arc<T: ?Sized> {
131 ptr: NonNull<ArcInner<T>>,
132 // NB: this informs dropck that objects of type `ArcInner<T>` may be used in `<Arc<T> as
133 // Drop>::drop`. Note that dropck already assumes that objects of type `T` may be used in
134 // `<Arc<T> as Drop>::drop` and the distinction between `T` and `ArcInner<T>` is not presently
135 // meaningful with respect to dropck - but this may change in the future so this is left here
136 // out of an abundance of caution.
137 //
138 // See https://doc.rust-lang.org/nomicon/phantom-data.html#generic-parameters-and-drop-checking
139 // for more detail on the semantics of dropck in the presence of `PhantomData`.
140 _p: PhantomData<ArcInner<T>>,
141}
142
143#[pin_data]
144#[repr(C)]
145struct ArcInner<T: ?Sized> {
146 refcount: Opaque<bindings::refcount_t>,
147 data: T,
148}
149
150impl<T: ?Sized> ArcInner<T> {
151 /// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].
152 ///
153 /// # Safety
154 ///
155 /// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
156 /// not yet have been destroyed.
157 unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
158 let refcount_layout = Layout::new::<bindings::refcount_t>();
159 // SAFETY: The caller guarantees that the pointer is valid.
160 let val_layout = Layout::for_value(unsafe { &*ptr });
161 // SAFETY: We're computing the layout of a real struct that existed when compiling this
162 // binary, so its layout is not so large that it can trigger arithmetic overflow.
163 let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
164
165 // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
166 // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
167 //
168 // This is documented at:
169 // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
170 let ptr = ptr as *const ArcInner<T>;
171
172 // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
173 // pointer, since it originates from a previous call to `Arc::into_raw` on an `Arc` that is
174 // still valid.
175 let ptr = unsafe { ptr.byte_sub(val_offset) };
176
177 // SAFETY: The pointer can't be null since you can't have an `ArcInner<T>` value at the null
178 // address.
179 unsafe { NonNull::new_unchecked(ptr.cast_mut()) }
180 }
181}
182
183// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
184// dynamically-sized type (DST) `U`.
185#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
186impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}
187
188// This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
189#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
190impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<T> {}
191
192// SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because
193// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
194// `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` using a
195// mutable reference when the reference count reaches zero and `T` is dropped.
196unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
197
198// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync`
199// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
200// it needs `T` to be `Send` because any thread that has a `&Arc<T>` may clone it and get an
201// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference when
202// the reference count reaches zero and `T` is dropped.
203unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
204
205impl<T> Arc<T> {
206 /// Constructs a new reference counted instance of `T`.
207 pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
208 // INVARIANT: The refcount is initialised to a non-zero value.
209 let value = ArcInner {
210 // SAFETY: There are no safety requirements for this FFI call.
211 refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
212 data: contents,
213 };
214
215 let inner = KBox::new(value, flags)?;
216 let inner = KBox::leak(inner).into();
217
218 // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
219 // `Arc` object.
220 Ok(unsafe { Self::from_inner(inner) })
221 }
222}
223
224impl<T: ?Sized> Arc<T> {
225 /// Constructs a new [`Arc`] from an existing [`ArcInner`].
226 ///
227 /// # Safety
228 ///
229 /// The caller must ensure that `inner` points to a valid location and has a non-zero reference
230 /// count, one of which will be owned by the new [`Arc`] instance.
231 unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
232 // INVARIANT: By the safety requirements, the invariants hold.
233 Arc {
234 ptr: inner,
235 _p: PhantomData,
236 }
237 }
238
239 /// Convert the [`Arc`] into a raw pointer.
240 ///
241 /// The raw pointer has ownership of the refcount that this Arc object owned.
242 pub fn into_raw(self) -> *const T {
243 let ptr = self.ptr.as_ptr();
244 core::mem::forget(self);
245 // SAFETY: The pointer is valid.
246 unsafe { core::ptr::addr_of!((*ptr).data) }
247 }
248
249 /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
250 ///
251 /// # Safety
252 ///
253 /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
254 /// must not be called more than once for each previous call to [`Arc::into_raw`].
255 pub unsafe fn from_raw(ptr: *const T) -> Self {
256 // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
257 // `Arc` that is still valid.
258 let ptr = unsafe { ArcInner::container_of(ptr) };
259
260 // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
261 // reference count held then will be owned by the new `Arc` object.
262 unsafe { Self::from_inner(ptr) }
263 }
264
265 /// Returns an [`ArcBorrow`] from the given [`Arc`].
266 ///
267 /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
268 /// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
269 #[inline]
270 pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
271 // SAFETY: The constraint that the lifetime of the shared reference must outlive that of
272 // the returned `ArcBorrow` ensures that the object remains alive and that no mutable
273 // reference can be created.
274 unsafe { ArcBorrow::new(self.ptr) }
275 }
276
277 /// Compare whether two [`Arc`] pointers reference the same underlying object.
278 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
279 core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_ptr())
280 }
281
282 /// Converts this [`Arc`] into a [`UniqueArc`], or destroys it if it is not unique.
283 ///
284 /// When this destroys the `Arc`, it does so while properly avoiding races. This means that
285 /// this method will never call the destructor of the value.
286 ///
287 /// # Examples
288 ///
289 /// ```
290 /// use kernel::sync::{Arc, UniqueArc};
291 ///
292 /// let arc = Arc::new(42, GFP_KERNEL)?;
293 /// let unique_arc = arc.into_unique_or_drop();
294 ///
295 /// // The above conversion should succeed since refcount of `arc` is 1.
296 /// assert!(unique_arc.is_some());
297 ///
298 /// assert_eq!(*(unique_arc.unwrap()), 42);
299 ///
300 /// # Ok::<(), Error>(())
301 /// ```
302 ///
303 /// ```
304 /// use kernel::sync::{Arc, UniqueArc};
305 ///
306 /// let arc = Arc::new(42, GFP_KERNEL)?;
307 /// let another = arc.clone();
308 ///
309 /// let unique_arc = arc.into_unique_or_drop();
310 ///
311 /// // The above conversion should fail since refcount of `arc` is >1.
312 /// assert!(unique_arc.is_none());
313 ///
314 /// # Ok::<(), Error>(())
315 /// ```
316 pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
317 // We will manually manage the refcount in this method, so we disable the destructor.
318 let me = ManuallyDrop::new(self);
319 // SAFETY: We own a refcount, so the pointer is still valid.
320 let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
321
322 // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
323 // return without further touching the `Arc`. If the refcount reaches zero, then there are
324 // no other arcs, and we can create a `UniqueArc`.
325 //
326 // SAFETY: We own a refcount, so the pointer is not dangling.
327 let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
328 if is_zero {
329 // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
330 // accesses to the refcount.
331 unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
332
333 // INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
334 // must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
335 // their values.
336 Some(Pin::from(UniqueArc {
337 inner: ManuallyDrop::into_inner(me),
338 }))
339 } else {
340 None
341 }
342 }
343}
344
345impl<T: 'static> ForeignOwnable for Arc<T> {
346 type Borrowed<'a> = ArcBorrow<'a, T>;
347 type BorrowedMut<'a> = Self::Borrowed<'a>;
348
349 fn into_foreign(self) -> *mut crate::ffi::c_void {
350 ManuallyDrop::new(self).ptr.as_ptr().cast()
351 }
352
353 unsafe fn from_foreign(ptr: *mut crate::ffi::c_void) -> Self {
354 // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
355 // call to `Self::into_foreign`.
356 let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
357
358 // SAFETY: By the safety requirement of this function, we know that `ptr` came from
359 // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
360 // holds a reference count increment that is transferrable to us.
361 unsafe { Self::from_inner(inner) }
362 }
363
364 unsafe fn borrow<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
365 // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
366 // call to `Self::into_foreign`.
367 let inner = unsafe { NonNull::new_unchecked(ptr.cast::<ArcInner<T>>()) };
368
369 // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
370 // for the lifetime of the returned value.
371 unsafe { ArcBorrow::new(inner) }
372 }
373
374 unsafe fn borrow_mut<'a>(ptr: *mut crate::ffi::c_void) -> ArcBorrow<'a, T> {
375 // SAFETY: The safety requirements for `borrow_mut` are a superset of the safety
376 // requirements for `borrow`.
377 unsafe { Self::borrow(ptr) }
378 }
379}
380
381impl<T: ?Sized> Deref for Arc<T> {
382 type Target = T;
383
384 fn deref(&self) -> &Self::Target {
385 // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
386 // safe to dereference it.
387 unsafe { &self.ptr.as_ref().data }
388 }
389}
390
391impl<T: ?Sized> AsRef<T> for Arc<T> {
392 fn as_ref(&self) -> &T {
393 self.deref()
394 }
395}
396
397impl<T: ?Sized> Clone for Arc<T> {
398 fn clone(&self) -> Self {
399 // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
400 // safe to dereference it.
401 let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
402
403 // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
404 // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
405 // safe to increment the refcount.
406 unsafe { bindings::refcount_inc(refcount) };
407
408 // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
409 unsafe { Self::from_inner(self.ptr) }
410 }
411}
412
413impl<T: ?Sized> Drop for Arc<T> {
414 fn drop(&mut self) {
415 // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
416 // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
417 // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
418 // freed/invalid memory as long as it is never dereferenced.
419 let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
420
421 // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
422 // this instance is being dropped, so the broken invariant is not observable.
423 // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
424 let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
425 if is_zero {
426 // The count reached zero, we must free the memory.
427 //
428 // SAFETY: The pointer was initialised from the result of `KBox::leak`.
429 unsafe { drop(KBox::from_raw(self.ptr.as_ptr())) };
430 }
431 }
432}
433
434impl<T: ?Sized> From<UniqueArc<T>> for Arc<T> {
435 fn from(item: UniqueArc<T>) -> Self {
436 item.inner
437 }
438}
439
440impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
441 fn from(item: Pin<UniqueArc<T>>) -> Self {
442 // SAFETY: The type invariants of `Arc` guarantee that the data is pinned.
443 unsafe { Pin::into_inner_unchecked(item).inner }
444 }
445}
446
447/// A borrowed reference to an [`Arc`] instance.
448///
449/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
450/// to use just `&T`, which we can trivially get from an [`Arc<T>`] instance.
451///
452/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
453/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
454/// to a pointer ([`Arc<T>`]) to the object (`T`). An [`ArcBorrow`] eliminates this double
455/// indirection while still allowing one to increment the refcount and getting an [`Arc<T>`] when/if
456/// needed.
457///
458/// # Invariants
459///
460/// There are no mutable references to the underlying [`Arc`], and it remains valid for the
461/// lifetime of the [`ArcBorrow`] instance.
462///
463/// # Example
464///
465/// ```
466/// use kernel::sync::{Arc, ArcBorrow};
467///
468/// struct Example;
469///
470/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
471/// e.into()
472/// }
473///
474/// let obj = Arc::new(Example, GFP_KERNEL)?;
475/// let cloned = do_something(obj.as_arc_borrow());
476///
477/// // Assert that both `obj` and `cloned` point to the same underlying object.
478/// assert!(core::ptr::eq(&*obj, &*cloned));
479/// # Ok::<(), Error>(())
480/// ```
481///
482/// Using `ArcBorrow<T>` as the type of `self`:
483///
484/// ```
485/// use kernel::sync::{Arc, ArcBorrow};
486///
487/// struct Example {
488/// a: u32,
489/// b: u32,
490/// }
491///
492/// impl Example {
493/// fn use_reference(self: ArcBorrow<'_, Self>) {
494/// // ...
495/// }
496/// }
497///
498/// let obj = Arc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
499/// obj.as_arc_borrow().use_reference();
500/// # Ok::<(), Error>(())
501/// ```
502#[repr(transparent)]
503#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
504pub struct ArcBorrow<'a, T: ?Sized + 'a> {
505 inner: NonNull<ArcInner<T>>,
506 _p: PhantomData<&'a ()>,
507}
508
509// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
510// `ArcBorrow<U>`.
511#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
512impl<T: ?Sized + core::marker::Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
513 for ArcBorrow<'_, T>
514{
515}
516
517impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
518 fn clone(&self) -> Self {
519 *self
520 }
521}
522
523impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}
524
525impl<T: ?Sized> ArcBorrow<'_, T> {
526 /// Creates a new [`ArcBorrow`] instance.
527 ///
528 /// # Safety
529 ///
530 /// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
531 /// 1. That `inner` remains valid;
532 /// 2. That no mutable references to `inner` are created.
533 unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
534 // INVARIANT: The safety requirements guarantee the invariants.
535 Self {
536 inner,
537 _p: PhantomData,
538 }
539 }
540
541 /// Creates an [`ArcBorrow`] to an [`Arc`] that has previously been deconstructed with
542 /// [`Arc::into_raw`].
543 ///
544 /// # Safety
545 ///
546 /// * The provided pointer must originate from a call to [`Arc::into_raw`].
547 /// * For the duration of the lifetime annotated on this `ArcBorrow`, the reference count must
548 /// not hit zero.
549 /// * For the duration of the lifetime annotated on this `ArcBorrow`, there must not be a
550 /// [`UniqueArc`] reference to this value.
551 pub unsafe fn from_raw(ptr: *const T) -> Self {
552 // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
553 // `Arc` that is still valid.
554 let ptr = unsafe { ArcInner::container_of(ptr) };
555
556 // SAFETY: The caller promises that the value remains valid since the reference count must
557 // not hit zero, and no mutable reference will be created since that would involve a
558 // `UniqueArc`.
559 unsafe { Self::new(ptr) }
560 }
561}
562
563impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
564 fn from(b: ArcBorrow<'_, T>) -> Self {
565 // SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
566 // guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
567 // increment.
568 ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
569 .deref()
570 .clone()
571 }
572}
573
574impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
575 type Target = T;
576
577 fn deref(&self) -> &Self::Target {
578 // SAFETY: By the type invariant, the underlying object is still alive with no mutable
579 // references to it, so it is safe to create a shared reference.
580 unsafe { &self.inner.as_ref().data }
581 }
582}
583
584/// A refcounted object that is known to have a refcount of 1.
585///
586/// It is mutable and can be converted to an [`Arc`] so that it can be shared.
587///
588/// # Invariants
589///
590/// `inner` always has a reference count of 1.
591///
592/// # Examples
593///
594/// In the following example, we make changes to the inner object before turning it into an
595/// `Arc<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()`
596/// cannot fail.
597///
598/// ```
599/// use kernel::sync::{Arc, UniqueArc};
600///
601/// struct Example {
602/// a: u32,
603/// b: u32,
604/// }
605///
606/// fn test() -> Result<Arc<Example>> {
607/// let mut x = UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?;
608/// x.a += 1;
609/// x.b += 1;
610/// Ok(x.into())
611/// }
612///
613/// # test().unwrap();
614/// ```
615///
616/// In the following example we first allocate memory for a refcounted `Example` but we don't
617/// initialise it on allocation. We do initialise it later with a call to [`UniqueArc::write`],
618/// followed by a conversion to `Arc<Example>`. This is particularly useful when allocation happens
619/// in one context (e.g., sleepable) and initialisation in another (e.g., atomic):
620///
621/// ```
622/// use kernel::sync::{Arc, UniqueArc};
623///
624/// struct Example {
625/// a: u32,
626/// b: u32,
627/// }
628///
629/// fn test() -> Result<Arc<Example>> {
630/// let x = UniqueArc::new_uninit(GFP_KERNEL)?;
631/// Ok(x.write(Example { a: 10, b: 20 }).into())
632/// }
633///
634/// # test().unwrap();
635/// ```
636///
637/// In the last example below, the caller gets a pinned instance of `Example` while converting to
638/// `Arc<Example>`; this is useful in scenarios where one needs a pinned reference during
639/// initialisation, for example, when initialising fields that are wrapped in locks.
640///
641/// ```
642/// use kernel::sync::{Arc, UniqueArc};
643///
644/// struct Example {
645/// a: u32,
646/// b: u32,
647/// }
648///
649/// fn test() -> Result<Arc<Example>> {
650/// let mut pinned = Pin::from(UniqueArc::new(Example { a: 10, b: 20 }, GFP_KERNEL)?);
651/// // We can modify `pinned` because it is `Unpin`.
652/// pinned.as_mut().a += 1;
653/// Ok(pinned.into())
654/// }
655///
656/// # test().unwrap();
657/// ```
658pub struct UniqueArc<T: ?Sized> {
659 inner: Arc<T>,
660}
661
662impl<T> UniqueArc<T> {
663 /// Tries to allocate a new [`UniqueArc`] instance.
664 pub fn new(value: T, flags: Flags) -> Result<Self, AllocError> {
665 Ok(Self {
666 // INVARIANT: The newly-created object has a refcount of 1.
667 inner: Arc::new(value, flags)?,
668 })
669 }
670
671 /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
672 pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError> {
673 // INVARIANT: The refcount is initialised to a non-zero value.
674 let inner = KBox::try_init::<AllocError>(
675 try_init!(ArcInner {
676 // SAFETY: There are no safety requirements for this FFI call.
677 refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
678 data <- init::uninit::<T, AllocError>(),
679 }? AllocError),
680 flags,
681 )?;
682 Ok(UniqueArc {
683 // INVARIANT: The newly-created object has a refcount of 1.
684 // SAFETY: The pointer from the `KBox` is valid.
685 inner: unsafe { Arc::from_inner(KBox::leak(inner).into()) },
686 })
687 }
688}
689
690impl<T> UniqueArc<MaybeUninit<T>> {
691 /// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
692 pub fn write(mut self, value: T) -> UniqueArc<T> {
693 self.deref_mut().write(value);
694 // SAFETY: We just wrote the value to be initialized.
695 unsafe { self.assume_init() }
696 }
697
698 /// Unsafely assume that `self` is initialized.
699 ///
700 /// # Safety
701 ///
702 /// The caller guarantees that the value behind this pointer has been initialized. It is
703 /// *immediate* UB to call this when the value is not initialized.
704 pub unsafe fn assume_init(self) -> UniqueArc<T> {
705 let inner = ManuallyDrop::new(self).inner.ptr;
706 UniqueArc {
707 // SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
708 // dropped). The types are compatible because `MaybeUninit<T>` is compatible with `T`.
709 inner: unsafe { Arc::from_inner(inner.cast()) },
710 }
711 }
712
713 /// Initialize `self` using the given initializer.
714 pub fn init_with<E>(mut self, init: impl Init<T, E>) -> core::result::Result<UniqueArc<T>, E> {
715 // SAFETY: The supplied pointer is valid for initialization.
716 match unsafe { init.__init(self.as_mut_ptr()) } {
717 // SAFETY: Initialization completed successfully.
718 Ok(()) => Ok(unsafe { self.assume_init() }),
719 Err(err) => Err(err),
720 }
721 }
722
723 /// Pin-initialize `self` using the given pin-initializer.
724 pub fn pin_init_with<E>(
725 mut self,
726 init: impl PinInit<T, E>,
727 ) -> core::result::Result<Pin<UniqueArc<T>>, E> {
728 // SAFETY: The supplied pointer is valid for initialization and we will later pin the value
729 // to ensure it does not move.
730 match unsafe { init.__pinned_init(self.as_mut_ptr()) } {
731 // SAFETY: Initialization completed successfully.
732 Ok(()) => Ok(unsafe { self.assume_init() }.into()),
733 Err(err) => Err(err),
734 }
735 }
736}
737
738impl<T: ?Sized> From<UniqueArc<T>> for Pin<UniqueArc<T>> {
739 fn from(obj: UniqueArc<T>) -> Self {
740 // SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueArc<T>>` (unless `T`
741 // is `Unpin`), so it is ok to convert it to `Pin<UniqueArc<T>>`.
742 unsafe { Pin::new_unchecked(obj) }
743 }
744}
745
746impl<T: ?Sized> Deref for UniqueArc<T> {
747 type Target = T;
748
749 fn deref(&self) -> &Self::Target {
750 self.inner.deref()
751 }
752}
753
754impl<T: ?Sized> DerefMut for UniqueArc<T> {
755 fn deref_mut(&mut self) -> &mut Self::Target {
756 // SAFETY: By the `Arc` type invariant, there is necessarily a reference to the object, so
757 // it is safe to dereference it. Additionally, we know there is only one reference when
758 // it's inside a `UniqueArc`, so it is safe to get a mutable reference.
759 unsafe { &mut self.inner.ptr.as_mut().data }
760 }
761}
762
763impl<T: fmt::Display + ?Sized> fmt::Display for UniqueArc<T> {
764 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
765 fmt::Display::fmt(self.deref(), f)
766 }
767}
768
769impl<T: fmt::Display + ?Sized> fmt::Display for Arc<T> {
770 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
771 fmt::Display::fmt(self.deref(), f)
772 }
773}
774
775impl<T: fmt::Debug + ?Sized> fmt::Debug for UniqueArc<T> {
776 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
777 fmt::Debug::fmt(self.deref(), f)
778 }
779}
780
781impl<T: fmt::Debug + ?Sized> fmt::Debug for Arc<T> {
782 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
783 fmt::Debug::fmt(self.deref(), f)
784 }
785}