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// SPDX-License-Identifier: GPL-2.0
//! Revocable objects.
//!
//! The [`Revocable`] type wraps other types and allows access to them to be revoked. The existence
//! of a [`RevocableGuard`] ensures that objects remain valid.
use crate::{bindings, sync::rcu};
use core::{
cell::UnsafeCell,
marker::PhantomData,
mem::MaybeUninit,
ops::Deref,
ptr::drop_in_place,
sync::atomic::{fence, AtomicBool, AtomicU32, Ordering},
};
/// An object that can become inaccessible at runtime.
///
/// Once access is revoked and all concurrent users complete (i.e., all existing instances of
/// [`RevocableGuard`] are dropped), the wrapped object is also dropped.
///
/// # Examples
///
/// ```
/// # use kernel::revocable::Revocable;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn add_two(v: &Revocable<Example>) -> Option<u32> {
/// let guard = v.try_access()?;
/// Some(guard.a + guard.b)
/// }
///
/// let v = Revocable::new(Example { a: 10, b: 20 });
/// assert_eq!(add_two(&v), Some(30));
/// v.revoke();
/// assert_eq!(add_two(&v), None);
/// ```
///
/// Sample example as above, but explicitly using the rcu read side lock.
///
/// ```
/// # use kernel::revocable::Revocable;
/// use kernel::sync::rcu;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn add_two(v: &Revocable<Example>) -> Option<u32> {
/// let guard = rcu::read_lock();
/// let e = v.try_access_with_guard(&guard)?;
/// Some(e.a + e.b)
/// }
///
/// let v = Revocable::new(Example { a: 10, b: 20 });
/// assert_eq!(add_two(&v), Some(30));
/// v.revoke();
/// assert_eq!(add_two(&v), None);
/// ```
pub struct Revocable<T> {
is_available: AtomicBool,
data: MaybeUninit<UnsafeCell<T>>,
}
// SAFETY: `Revocable` is `Send` if the wrapped object is also `Send`. This is because while the
// functionality exposed by `Revocable` can be accessed from any thread/CPU, it is possible that
// this isn't supported by the wrapped object.
unsafe impl<T: Send> Send for Revocable<T> {}
// SAFETY: `Revocable` is `Sync` if the wrapped object is both `Send` and `Sync`. We require `Send`
// from the wrapped object as well because of `Revocable::revoke`, which can trigger the `Drop`
// implementation of the wrapped object from an arbitrary thread.
unsafe impl<T: Sync + Send> Sync for Revocable<T> {}
impl<T> Revocable<T> {
/// Creates a new revocable instance of the given data.
pub const fn new(data: T) -> Self {
Self {
is_available: AtomicBool::new(true),
data: MaybeUninit::new(UnsafeCell::new(data)),
}
}
/// Tries to access the \[revocable\] wrapped object.
///
/// Returns `None` if the object has been revoked and is therefore no longer accessible.
///
/// Returns a guard that gives access to the object otherwise; the object is guaranteed to
/// remain accessible while the guard is alive. In such cases, callers are not allowed to sleep
/// because another CPU may be waiting to complete the revocation of this object.
pub fn try_access(&self) -> Option<RevocableGuard<'_, T>> {
let guard = rcu::read_lock();
if self.is_available.load(Ordering::Relaxed) {
// SAFETY: Since `self.is_available` is true, data is initialised and has to remain
// valid because the RCU read side lock prevents it from being dropped.
Some(unsafe { RevocableGuard::new(self.data.assume_init_ref().get(), guard) })
} else {
None
}
}
/// Tries to access the \[revocable\] wrapped object.
///
/// Returns `None` if the object has been revoked and is therefore no longer accessible.
///
/// Returns a shared reference to the object otherwise; the object is guaranteed to
/// remain accessible while the rcu read side guard is alive. In such cases, callers are not
/// allowed to sleep because another CPU may be waiting to complete the revocation of this
/// object.
pub fn try_access_with_guard<'a>(&'a self, _guard: &'a rcu::Guard) -> Option<&'a T> {
if self.is_available.load(Ordering::Relaxed) {
// SAFETY: Since `self.is_available` is true, data is initialised and has to remain
// valid because the RCU read side lock prevents it from being dropped.
Some(unsafe { &*self.data.assume_init_ref().get() })
} else {
None
}
}
/// Revokes access to and drops the wrapped object.
///
/// Access to the object is revoked immediately to new callers of [`Revocable::try_access`]. If
/// there are concurrent users of the object (i.e., ones that called [`Revocable::try_access`]
/// beforehand and still haven't dropped the returned guard), this function waits for the
/// concurrent access to complete before dropping the wrapped object.
pub fn revoke(&self) {
if self
.is_available
.compare_exchange(true, false, Ordering::Relaxed, Ordering::Relaxed)
.is_ok()
{
// SAFETY: Just an FFI call, there are no further requirements.
unsafe { bindings::synchronize_rcu() };
// SAFETY: We know `self.data` is valid because only one CPU can succeed the
// `compare_exchange` above that takes `is_available` from `true` to `false`.
unsafe { drop_in_place(self.data.assume_init_ref().get()) };
}
}
}
impl<T> Drop for Revocable<T> {
fn drop(&mut self) {
// Drop only if the data hasn't been revoked yet (in which case it has already been
// dropped).
if *self.is_available.get_mut() {
// SAFETY: We know `self.data` is valid because no other CPU has changed
// `is_available` to `false` yet, and no other CPU can do it anymore because this CPU
// holds the only reference (mutable) to `self` now.
unsafe { drop_in_place(self.data.assume_init_ref().get()) };
}
}
}
/// A guard that allows access to a revocable object and keeps it alive.
///
/// CPUs may not sleep while holding on to [`RevocableGuard`] because it's in atomic context
/// holding the RCU read-side lock.
///
/// # Invariants
///
/// The RCU read-side lock is held while the guard is alive.
pub struct RevocableGuard<'a, T> {
data_ref: *const T,
_rcu_guard: rcu::Guard,
_p: PhantomData<&'a ()>,
}
impl<T> RevocableGuard<'_, T> {
fn new(data_ref: *const T, rcu_guard: rcu::Guard) -> Self {
Self {
data_ref,
_rcu_guard: rcu_guard,
_p: PhantomData,
}
}
}
impl<T> Deref for RevocableGuard<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: By the type invariants, we hold the rcu read-side lock, so the object is
// guaranteed to remain valid.
unsafe { &*self.data_ref }
}
}
/// An object that can become inaccessible at runtime.
///
/// Once access is revoked and all concurrent users complete (i.e., all existing instances of
/// [`AsyncRevocableGuard`] are dropped), the wrapped object is also dropped.
///
/// Unlike [`Revocable`], [`AsyncRevocable`] does not wait for concurrent users of the wrapped
/// object to finish before [`AsyncRevocable::revoke`] completes -- thus the async qualifier. This
/// has the advantage of not requiring RCU locks or waits of any kind.
///
/// # Examples
///
/// ```
/// # use kernel::revocable::AsyncRevocable;
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// fn add_two(v: &AsyncRevocable<Example>) -> Option<u32> {
/// let guard = v.try_access()?;
/// Some(guard.a + guard.b)
/// }
///
/// let v = AsyncRevocable::new(Example { a: 10, b: 20 });
/// assert_eq!(add_two(&v), Some(30));
/// v.revoke();
/// assert_eq!(add_two(&v), None);
/// ```
///
/// Example where revocation happens while there is a user:
///
/// ```
/// # use kernel::revocable::AsyncRevocable;
/// use core::sync::atomic::{AtomicBool, Ordering};
///
/// struct Example {
/// a: u32,
/// b: u32,
/// }
///
/// static DROPPED: AtomicBool = AtomicBool::new(false);
///
/// impl Drop for Example {
/// fn drop(&mut self) {
/// DROPPED.store(true, Ordering::Relaxed);
/// }
/// }
///
/// fn add_two(v: &AsyncRevocable<Example>) -> Option<u32> {
/// let guard = v.try_access()?;
/// Some(guard.a + guard.b)
/// }
///
/// let v = AsyncRevocable::new(Example { a: 10, b: 20 });
/// assert_eq!(add_two(&v), Some(30));
///
/// let guard = v.try_access().unwrap();
/// assert!(!v.is_revoked());
/// assert!(!DROPPED.load(Ordering::Relaxed));
/// v.revoke();
/// assert!(!DROPPED.load(Ordering::Relaxed));
/// assert!(v.is_revoked());
/// assert!(v.try_access().is_none());
/// assert_eq!(guard.a + guard.b, 30);
/// drop(guard);
/// assert!(DROPPED.load(Ordering::Relaxed));
/// ```
pub struct AsyncRevocable<T> {
usage_count: AtomicU32,
data: MaybeUninit<UnsafeCell<T>>,
}
// SAFETY: `AsyncRevocable` is `Send` if the wrapped object is also `Send`. This is because while
// the functionality exposed by `AsyncRevocable` can be accessed from any thread/CPU, it is
// possible that this isn't supported by the wrapped object.
unsafe impl<T: Send> Send for AsyncRevocable<T> {}
// SAFETY: `AsyncRevocable` is `Sync` if the wrapped object is both `Send` and `Sync`. We require
// `Send` from the wrapped object as well because of `AsyncRevocable::revoke`, which can trigger
// the `Drop` implementation of the wrapped object from an arbitrary thread.
unsafe impl<T: Sync + Send> Sync for AsyncRevocable<T> {}
const REVOKED: u32 = 0x80000000;
const COUNT_MASK: u32 = !REVOKED;
const SATURATED_COUNT: u32 = REVOKED - 1;
impl<T> AsyncRevocable<T> {
/// Creates a new asynchronously revocable instance of the given data.
pub fn new(data: T) -> Self {
Self {
usage_count: AtomicU32::new(0),
data: MaybeUninit::new(UnsafeCell::new(data)),
}
}
/// Tries to access the \[revocable\] wrapped object.
///
/// Returns `None` if the object has been revoked and is therefore no longer accessible.
///
/// Returns a guard that gives access to the object otherwise; the object is guaranteed to
/// remain accessible while the guard is alive.
pub fn try_access(&self) -> Option<AsyncRevocableGuard<'_, T>> {
loop {
let count = self.usage_count.load(Ordering::Relaxed);
// Fail attempt to access if the object is already revoked.
if count & REVOKED != 0 {
return None;
}
// No need to increment if the count is saturated.
if count == SATURATED_COUNT
|| self
.usage_count
.compare_exchange(count, count + 1, Ordering::Relaxed, Ordering::Relaxed)
.is_ok()
{
return Some(AsyncRevocableGuard { revocable: self });
}
}
}
/// Revokes access to the protected object.
///
/// Returns `true` if access has been revoked, or `false` when the object has already been
/// revoked by a previous call to [`AsyncRevocable::revoke`].
///
/// This call is non-blocking, that is, no new users of the revocable object will be allowed,
/// but potential current users are able to continue to use it and the thread won't wait for
/// them to finish. In such cases, the object will be dropped when the last user completes.
pub fn revoke(&self) -> bool {
// Set the `REVOKED` bit.
//
// The acquire barrier matches up with the release when decrementing the usage count.
let prev = self.usage_count.fetch_or(REVOKED, Ordering::Acquire);
if prev & REVOKED != 0 {
// Another thread already revoked this object.
return false;
}
if prev == 0 {
// SAFETY: This thread just revoked the object and the usage count is zero, so the
// object is valid and there will be no future users.
unsafe { drop_in_place(UnsafeCell::raw_get(self.data.as_ptr())) };
}
true
}
/// Returns whether access to the object has been revoked.
pub fn is_revoked(&self) -> bool {
self.usage_count.load(Ordering::Relaxed) & REVOKED != 0
}
}
impl<T> Drop for AsyncRevocable<T> {
fn drop(&mut self) {
let count = *self.usage_count.get_mut();
if count != REVOKED {
// The object hasn't been dropped yet, so we do it now.
// This matches with the release when decrementing the usage count.
fence(Ordering::Acquire);
// SAFETY: Since `count` is does not indicate a count of 0 and the REVOKED bit set, the
// object is still valid.
unsafe { drop_in_place(UnsafeCell::raw_get(self.data.as_ptr())) };
}
}
}
/// A guard that allows access to a revocable object and keeps it alive.
///
/// # Invariants
///
/// The owner owns an increment on the usage count (which may have saturated it), which keeps the
/// revocable object alive.
pub struct AsyncRevocableGuard<'a, T> {
revocable: &'a AsyncRevocable<T>,
}
impl<T> Deref for AsyncRevocableGuard<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: The type invariants guarantee that the caller owns an increment.
unsafe { &*self.revocable.data.assume_init_ref().get() }
}
}
impl<T> Drop for AsyncRevocableGuard<'_, T> {
fn drop(&mut self) {
loop {
let count = self.revocable.usage_count.load(Ordering::Relaxed);
let actual_count = count & COUNT_MASK;
if actual_count == SATURATED_COUNT {
// The count is saturated, so we won't decrement (nor do we drop the object).
return;
}
if actual_count == 0 {
// Trying to underflow the count.
panic!("actual_count is zero");
}
// On success, we use release ordering, which matches with the acquire in one of the
// places where we drop the object, namely: below, in `AsyncRevocable::revoke`, or in
// `AsyncRevocable::drop`.
if self
.revocable
.usage_count
.compare_exchange(count, count - 1, Ordering::Release, Ordering::Relaxed)
.is_ok()
{
if count == 1 | REVOKED {
// `count` is now zero and it is revoked, so free it now.
// This matches with the release above (which may have happened in other
// threads concurrently).
fence(Ordering::Acquire);
// SAFETY: Since `count` was 1, the object is still alive.
unsafe { drop_in_place(UnsafeCell::raw_get(self.revocable.data.as_ptr())) };
}
return;
}
}
}
}