1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714
// SPDX-License-Identifier: GPL-2.0
//! A reference-counted pointer.
//!
//! This module implements a way for users to create reference-counted objects and pointers to
//! them. Such a pointer automatically increments and decrements the count, and drops the
//! underlying object when it reaches zero. It is also safe to use concurrently from multiple
//! threads.
//!
//! It is different from the standard library's [`Arc`] in a few ways:
//! 1. It is backed by the kernel's `refcount_t` type.
//! 2. It does not support weak references, which allows it to be half the size.
//! 3. It saturates the reference count instead of aborting when it goes over a threshold.
//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
//!
//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
use crate::{bindings, error::code::*, types::ForeignOwnable, Error, Opaque, Result};
use alloc::{
alloc::{alloc, dealloc},
vec::Vec,
};
use core::{
alloc::Layout,
convert::{AsRef, TryFrom},
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
ops::{Deref, DerefMut},
pin::Pin,
ptr::{self, NonNull},
};
/// A reference-counted pointer to an instance of `T`.
///
/// The reference count is incremented when new instances of [`Arc`] are created, and decremented
/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped.
///
/// # Invariants
///
/// The reference count on an instance of [`Arc`] is always non-zero.
/// The object pointed to by [`Arc`] is always pinned.
///
/// # Examples
///
/// ```
/// use kernel::sync::Arc;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// // Create a ref-counted instance of `Example`.
/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
///
/// // Get a new pointer to `obj` and increment the refcount.
/// let cloned = obj.clone();
///
/// // Assert that both `obj` and `cloned` point to the same underlying object.
/// assert!(core::ptr::eq(&*obj, &*cloned));
///
/// // Destroy `obj` and decrement its refcount.
/// drop(obj);
///
/// // Check that the values are still accessible through `cloned`.
/// assert_eq!(cloned.a, 10);
/// assert_eq!(cloned.b, 20);
///
/// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
///
/// # Ok::<(), Error>(())
/// ```
///
/// Using `Arc<T>` as the type of `self`:
///
/// ```
/// use kernel::sync::Arc;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// impl Example {
/// fn take_over(self: Arc<Self>) {
/// // ...
/// }
///
/// fn use_reference(self: &Arc<Self>) {
/// // ...
/// }
/// }
///
/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
/// obj.use_reference();
/// obj.take_over();
///
/// # Ok::<(), Error>(())
/// ```
///
/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// trait MyTrait {
/// // Trait has a function whose `self` type is `Arc<Self>`.
/// fn example1(self: Arc<Self>) {}
///
/// // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
/// fn example2(self: ArcBorrow<'_, Self>) {}
/// }
///
/// struct Example;
/// impl MyTrait for Example {}
///
/// // `obj` has type `Arc<Example>`.
/// let obj: Arc<Example> = Arc::try_new(Example)?;
///
/// // `coerced` has type `Arc<dyn MyTrait>`.
/// let coerced: Arc<dyn MyTrait> = obj;
///
/// # Ok::<(), Error>(())
/// ```
pub struct Arc<T: ?Sized> {
ptr: NonNull<ArcInner<T>>,
_p: PhantomData<ArcInner<T>>,
}
#[repr(C)]
struct ArcInner<T: ?Sized> {
refcount: Opaque<bindings::refcount_t>,
data: T,
}
// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
// dynamically-sized type (DST) `U`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}
// This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<T> {}
// SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because
// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
// `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` directly, for
// example, when the reference count reaches zero and `T` is dropped.
unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync` for the
// same reason as above. `T` needs to be `Send` as well because a thread can clone an `&Arc<T>`
// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above.
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
impl<T> Arc<T> {
/// Constructs a new reference counted instance of `T`.
pub fn try_new(contents: T) -> Result<Self> {
let layout = Layout::new::<ArcInner<T>>();
// SAFETY: The layout size is guaranteed to be non-zero because `ArcInner` contains the
// reference count.
let inner = NonNull::new(unsafe { alloc(layout) })
.ok_or(ENOMEM)?
.cast::<ArcInner<T>>();
// INVARIANT: The refcount is initialised to a non-zero value.
let value = ArcInner {
refcount: Opaque::new(new_refcount()),
data: contents,
};
// SAFETY: `inner` is writable and properly aligned.
unsafe { inner.as_ptr().write(value) };
// SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
// `Arc` object.
Ok(unsafe { Self::from_inner(inner) })
}
}
impl<T: ?Sized> Arc<T> {
/// Constructs a new [`Arc`] from an existing [`ArcInner`].
///
/// # Safety
///
/// The caller must ensure that `inner` points to a valid location and has a non-zero reference
/// count, one of which will be owned by the new [`Arc`] instance.
unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
// INVARIANT: By the safety requirements, the invariants hold.
Arc {
ptr: inner,
_p: PhantomData,
}
}
/// Determines if two reference-counted pointers point to the same underlying instance of `T`.
pub fn ptr_eq(a: &Self, b: &Self) -> bool {
ptr::eq(a.ptr.as_ptr(), b.ptr.as_ptr())
}
/// Deconstructs a [`Arc`] object into a raw pointer.
///
/// It can be reconstructed once via [`Arc::from_raw`].
pub fn into_raw(obj: Self) -> *const T {
let ret = &*obj as *const T;
core::mem::forget(obj);
ret
}
/// Recreates a [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
///
/// This code relies on the `repr(C)` layout of structs as described in
/// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
///
/// # Safety
///
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
/// can only be called once for each previous call to [`Arc::into_raw`].
pub unsafe fn from_raw(ptr: *const T) -> Self {
// SAFETY: The safety requirement ensures that the pointer is valid.
let align = core::mem::align_of_val(unsafe { &*ptr });
let offset = Layout::new::<ArcInner<()>>()
.align_to(align)
.unwrap()
.pad_to_align()
.size();
// SAFETY: The pointer is in bounds because by the safety requirements `ptr` came from
// `Arc::into_raw`, so it is a pointer `offset` bytes from the beginning of the allocation.
let data = unsafe { (ptr as *const u8).sub(offset) };
let metadata = ptr::metadata(ptr as *const ArcInner<T>);
let ptr = ptr::from_raw_parts_mut(data as _, metadata);
// SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
// reference count held then will be owned by the new `Arc` object.
unsafe { Self::from_inner(NonNull::new(ptr).unwrap()) }
}
/// Returns a [`ArcBorrow`] from the given [`Arc`].
///
/// This is useful when the argument of a function call is a [`ArcBorrow`] (e.g., in a method
/// receiver), but we have a [`Arc`] instead. Getting a [`ArcBorrow`] is free when optimised.
#[inline]
pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
// SAFETY: The constraint that the lifetime of the shared reference must outlive that of
// the returned `ArcBorrow` ensures that the object remains alive and that no mutable
// reference can be created.
unsafe { ArcBorrow::new(self.ptr) }
}
}
impl<T: 'static> ForeignOwnable for Arc<T> {
type Borrowed<'a> = ArcBorrow<'a, T>;
fn into_foreign(self) -> *const core::ffi::c_void {
ManuallyDrop::new(self).ptr.as_ptr() as _
}
unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
// SAFETY: By the safety requirement of this function, we know that `ptr` came from
// a previous call to `Arc::into_foreign`.
let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
// SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
// for the lifetime of the returned value. Additionally, the safety requirements of
// `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
unsafe { ArcBorrow::new(inner) }
}
unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
// SAFETY: By the safety requirement of this function, we know that `ptr` came from
// a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
// holds a reference count increment that is transferrable to us.
unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
}
}
impl<T: ?Sized> Deref for Arc<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to dereference it.
unsafe { &self.ptr.as_ref().data }
}
}
impl<T: ?Sized> Clone for Arc<T> {
fn clone(&self) -> Self {
// INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to increment the refcount.
unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
// SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
unsafe { Self::from_inner(self.ptr) }
}
}
impl<T: ?Sized> AsRef<T> for Arc<T> {
fn as_ref(&self) -> &T {
// SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
// safe to dereference it.
unsafe { &self.ptr.as_ref().data }
}
}
impl<T: ?Sized> Drop for Arc<T> {
fn drop(&mut self) {
// SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
// touch `refcount` after it's decremented to a non-zero value because another thread/CPU
// may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
// freed/invalid memory as long as it is never dereferenced.
let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
// INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
// this instance is being dropped, so the broken invariant is not observable.
// SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
if is_zero {
// The count reached zero, we must free the memory.
// SAFETY: This thread holds the only remaining reference to `self`, so it is safe to
// get a mutable reference to it.
let inner = unsafe { self.ptr.as_mut() };
let layout = Layout::for_value(inner);
// SAFETY: The value stored in inner is valid.
unsafe { core::ptr::drop_in_place(inner) };
// SAFETY: The pointer was initialised from the result of a call to `alloc`.
unsafe { dealloc(self.ptr.cast().as_ptr(), layout) };
}
}
}
impl<T> TryFrom<Vec<T>> for Arc<[T]> {
type Error = Error;
fn try_from(mut v: Vec<T>) -> Result<Self> {
let value_layout = Layout::array::<T>(v.len())?;
let layout = Layout::new::<ArcInner<()>>()
.extend(value_layout)?
.0
.pad_to_align();
// SAFETY: The layout size is guaranteed to be non-zero because `ArcInner` contains the
// reference count.
let ptr = NonNull::new(unsafe { alloc(layout) }).ok_or(ENOMEM)?;
let inner =
core::ptr::slice_from_raw_parts_mut(ptr.as_ptr() as _, v.len()) as *mut ArcInner<[T]>;
// SAFETY: Just an FFI call that returns a `refcount_t` initialised to 1.
let count = Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) });
// SAFETY: `inner.refcount` is writable and properly aligned.
unsafe { core::ptr::addr_of_mut!((*inner).refcount).write(count) };
// SAFETY: The contents of `v` as readable and properly aligned; `inner.data` is writable
// and properly aligned. There is no overlap between the two because `inner` is a new
// allocation.
unsafe {
core::ptr::copy_nonoverlapping(
v.as_ptr(),
core::ptr::addr_of_mut!((*inner).data) as *mut [T] as *mut T,
v.len(),
)
};
// SAFETY: We're setting the new length to zero, so it is <= to capacity, and old_len..0 is
// an empty range (so satisfies vacuously the requirement of being initialised).
unsafe { v.set_len(0) };
// SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
// `Arc` object.
Ok(unsafe { Self::from_inner(NonNull::new(inner).unwrap()) })
}
}
impl<T: ?Sized> From<UniqueArc<T>> for Arc<T> {
fn from(item: UniqueArc<T>) -> Self {
item.inner
}
}
impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
fn from(item: Pin<UniqueArc<T>>) -> Self {
// SAFETY: The type invariants of `Arc` guarantee that the data is pinned.
unsafe { Pin::into_inner_unchecked(item).inner }
}
}
/// A borrowed reference to an [`Arc`] instance.
///
/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
/// to use just `&T`, which we can trivially get from an `Arc<T>` instance.
///
/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
/// to a pointer (`Arc<T>`) to the object (`T`). An [`ArcBorrow`] eliminates this double
/// indirection while still allowing one to increment the refcount and getting an `Arc<T>` when/if
/// needed.
///
/// # Invariants
///
/// There are no mutable references to the underlying [`Arc`], and it remains valid for the
/// lifetime of the [`ArcBorrow`] instance.
///
/// # Example
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// struct Example;
///
/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
/// e.into()
/// }
///
/// let obj = Arc::try_new(Example)?;
/// let cloned = do_something(obj.as_arc_borrow());
///
/// // Assert that both `obj` and `cloned` point to the same underlying object.
/// assert!(core::ptr::eq(&*obj, &*cloned));
///
/// # Ok::<(), Error>(())
/// ```
///
/// Using `ArcBorrow<T>` as the type of `self`:
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// impl Example {
/// fn use_reference(self: ArcBorrow<'_, Self>) {
/// // ...
/// }
/// }
///
/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
/// obj.as_arc_borrow().use_reference();
///
/// # Ok::<(), Error>(())
/// ```
pub struct ArcBorrow<'a, T: ?Sized + 'a> {
inner: NonNull<ArcInner<T>>,
_p: PhantomData<&'a ()>,
}
// This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}
// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
// `ArcBorrow<U>`.
impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
for ArcBorrow<'_, T>
{
}
impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
fn clone(&self) -> Self {
*self
}
}
impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}
impl<T: ?Sized> ArcBorrow<'_, T> {
/// Creates a new [`ArcBorrow`] instance.
///
/// # Safety
///
/// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
/// 1. That `inner` remains valid;
/// 2. That no mutable references to `inner` are created.
unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
// INVARIANT: The safety requirements guarantee the invariants.
Self {
inner,
_p: PhantomData,
}
}
}
impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
fn from(b: ArcBorrow<'_, T>) -> Self {
// SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
// guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
// increment.
ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
.deref()
.clone()
}
}
impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: By the type invariant, the underlying object is still alive with no mutable
// references to it, so it is safe to create a shared reference.
unsafe { &self.inner.as_ref().data }
}
}
/// A refcounted object that is known to have a refcount of 1.
///
/// It is mutable and can be converted to an [`Arc`] so that it can be shared.
///
/// # Invariants
///
/// `inner` always has a reference count of 1.
///
/// # Examples
///
/// In the following example, we make changes to the inner object before turning it into an
/// `Arc<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()`
/// cannot fail.
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let mut x = UniqueArc::try_new(Example { a: 10, b: 20 })?;
/// x.a += 1;
/// x.b += 1;
/// Ok(x.into())
/// }
///
/// # test().unwrap();
/// ```
///
/// In the following example we first allocate memory for a ref-counted `Example` but we don't
/// initialise it on allocation. We do initialise it later with a call to [`UniqueArc::write`],
/// followed by a conversion to `Arc<Example>`. This is particularly useful when allocation happens
/// in one context (e.g., sleepable) and initialisation in another (e.g., atomic):
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let x = UniqueArc::try_new_uninit()?;
/// Ok(x.write(Example { a: 10, b: 20 }).into())
/// }
///
/// # test().unwrap();
/// ```
///
/// In the last example below, the caller gets a pinned instance of `Example` while converting to
/// `Arc<Example>`; this is useful in scenarios where one needs a pinned reference during
/// initialisation, for example, when initialising fields that are wrapped in locks.
///
/// ```
/// use kernel::sync::{Arc, UniqueArc};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn test() -> Result<Arc<Example>> {
/// let mut pinned = Pin::from(UniqueArc::try_new(Example { a: 10, b: 20 })?);
/// // We can modify `pinned` because it is `Unpin`.
/// pinned.as_mut().a += 1;
/// Ok(pinned.into())
/// }
///
/// # test().unwrap();
/// ```
pub struct UniqueArc<T: ?Sized> {
inner: Arc<T>,
}
impl<T> UniqueArc<T> {
/// Tries to allocate a new [`UniqueArc`] instance.
pub fn try_new(value: T) -> Result<Self> {
Ok(Self {
// INVARIANT: The newly-created object has a ref-count of 1.
inner: Arc::try_new(value)?,
})
}
/// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> {
Ok(UniqueArc::<MaybeUninit<T>> {
// INVARIANT: The newly-created object has a ref-count of 1.
inner: Arc::try_new(MaybeUninit::uninit())?,
})
}
}
impl<T> UniqueArc<MaybeUninit<T>> {
/// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
pub fn write(mut self, value: T) -> UniqueArc<T> {
self.deref_mut().write(value);
let inner = ManuallyDrop::new(self).inner.ptr;
UniqueArc {
// SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
// dropped). The types are compatible because `MaybeUninit<T>` is compatible with `T`.
inner: unsafe { Arc::from_inner(inner.cast()) },
}
}
}
impl<T: ?Sized> From<UniqueArc<T>> for Pin<UniqueArc<T>> {
fn from(obj: UniqueArc<T>) -> Self {
// SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueArc<T>>` (unless `T`
// is `Unpin`), so it is ok to convert it to `Pin<UniqueArc<T>>`.
unsafe { Pin::new_unchecked(obj) }
}
}
impl<T: ?Sized> Deref for UniqueArc<T> {
type Target = T;
fn deref(&self) -> &Self::Target {
self.inner.deref()
}
}
impl<T: ?Sized> DerefMut for UniqueArc<T> {
fn deref_mut(&mut self) -> &mut Self::Target {
// SAFETY: By the `Arc` type invariant, there is necessarily a reference to the object, so
// it is safe to dereference it. Additionally, we know there is only one reference when
// it's inside a `UniqueArc`, so it is safe to get a mutable reference.
unsafe { &mut self.inner.ptr.as_mut().data }
}
}
/// Allows the creation of "reference-counted" globals.
///
/// This is achieved by biasing the refcount with +1, which ensures that the count never drops back
/// to zero (unless buggy unsafe code incorrectly decrements without owning an increment) and
/// therefore also ensures that `drop` is never called.
///
/// # Examples
///
/// ```
/// use kernel::sync::{Arc, ArcBorrow, StaticArc};
///
/// const VALUE: u32 = 10;
/// static SR: StaticArc<u32> = StaticArc::new(VALUE);
///
/// fn takes_ref_borrow(v: ArcBorrow<'_, u32>) {
/// assert_eq!(*v, VALUE);
/// }
///
/// fn takes_ref(v: Arc<u32>) {
/// assert_eq!(*v, VALUE);
/// }
///
/// takes_ref_borrow(SR.as_arc_borrow());
/// takes_ref(SR.as_arc_borrow().into());
/// ```
pub struct StaticArc<T: ?Sized> {
inner: ArcInner<T>,
}
// SAFETY: A `StaticArc<T>` is a `Arc<T>` declared statically, so we just use the same criteria for
// making it `Sync`.
unsafe impl<T: ?Sized + Sync + Send> Sync for StaticArc<T> {}
impl<T> StaticArc<T> {
/// Creates a new instance of a static "ref-counted" object.
pub const fn new(data: T) -> Self {
// INVARIANT: The refcount is initialised to a non-zero value.
Self {
inner: ArcInner {
refcount: Opaque::new(new_refcount()),
data,
},
}
}
}
impl<T: ?Sized> StaticArc<T> {
/// Creates a [`ArcBorrow`] instance from the given static object.
///
/// This requires a `'static` lifetime so that it can guarantee that the underlyling object
/// remains valid and is effectively pinned.
pub fn as_arc_borrow(&'static self) -> ArcBorrow<'static, T> {
// SAFETY: The static lifetime guarantees that the object remains valid. And the shared
// reference guarantees that no mutable references exist.
unsafe { ArcBorrow::new(NonNull::from(&self.inner)) }
}
}
/// Creates, from a const context, a new instance of `struct refcount_struct` with a refcount of 1.
///
/// ```
/// # // The test below is meant to ensure that `new_refcount` (which is const) mimics
/// # // `REFCOUNT_INIT`, which is written in C and thus can't be used in a const context.
/// # // TODO: Once `#[test]` is working, move this to a test and make `new_refcount` private.
/// # use kernel::bindings;
/// # // SAFETY: Just an FFI call that returns a `refcount_t` initialised to 1.
/// # let bindings::refcount_struct {
/// # refs: bindings::atomic_t { counter: a },
/// # } = unsafe { bindings::REFCOUNT_INIT(1) };
/// # let bindings::refcount_struct {
/// # refs: bindings::atomic_t { counter: b },
/// # } = kernel::sync::new_refcount();
/// # assert_eq!(a, b);
/// ```
pub const fn new_refcount() -> bindings::refcount_struct {
bindings::refcount_struct {
refs: bindings::atomic_t { counter: 1 },
}
}