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(I'm so sorry!) Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel os linux
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//! 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 pin_init::Wrapper;
use crate::{ prelude::*, sync::{ atomic::{ AtomicFlag, Relaxed, // }, rcu, // }, types::Opaque, //};use core::{ marker::PhantomData, ops::Deref, ptr::drop_in_place, //};
/// 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 = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap();/// 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 = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap();/// assert_eq!(add_two(&v), Some(30));/// v.revoke();/// assert_eq!(add_two(&v), None);/// ```#[pin_data(PinnedDrop)]pub struct Revocable<T> { is_available: AtomicFlag, #[pin] data: Opaque<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 fn new<E>(data: impl PinInit<T, E>) -> impl PinInit<Self, E> { try_pin_init!(Self { is_available: AtomicFlag::new(true), data <- Opaque::pin_init(data), }? E) }
/// 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(Relaxed) { // 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(RevocableGuard::new(self.data.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(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.get() }) } else { None } }
/// Tries to access the wrapped object and run a closure on it while the guard is held. /// /// This is a convenience method to run short non-sleepable code blocks while ensuring the /// guard is dropped afterwards. [`Self::try_access`] carries the risk that the caller will /// forget to explicitly drop that returned guard before calling sleepable code; this method /// adds an extra safety to make sure it doesn't happen. /// /// Returns [`None`] if the object has been revoked and is therefore no longer accessible, or /// the result of the closure wrapped in [`Some`]. If the closure returns a [`Result`] then the /// return type becomes `Option<Result<>>`, which can be inconvenient. Users are encouraged to /// define their own macro that turns the [`Option`] into a proper error code and flattens the /// inner result into it if it makes sense within their subsystem. pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> { self.try_access().map(|t| f(&*t)) }
/// Directly access the revocable wrapped object. /// /// # Safety /// /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked /// as long as the returned `&T` lives. pub unsafe fn access(&self) -> &T { // SAFETY: By the safety requirement of this function it is guaranteed that // `self.data.get()` is a valid pointer to an instance of `T`. unsafe { &*self.data.get() } }
/// # Safety /// /// Callers must ensure that there are no more concurrent users of the revocable object. unsafe fn revoke_internal<const SYNC: bool>(&self) -> bool { let revoke = self.is_available.xchg(false, Relaxed);
if revoke { if SYNC { rcu::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.get()) }; }
revoke }
/// Revokes access to and drops the wrapped object. /// /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`], /// expecting that there are no concurrent users of the object. /// /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked /// already. /// /// # Safety /// /// Callers must ensure that there are no more concurrent users of the revocable object. pub unsafe fn revoke_nosync(&self) -> bool { // SAFETY: By the safety requirement of this function, the caller ensures that nobody is // accessing the data anymore and hence we don't have to wait for the grace period to // finish. unsafe { self.revoke_internal::<false>() } }
/// 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. /// /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked /// already. pub fn revoke(&self) -> bool { // SAFETY: By passing `true` we ask `revoke_internal` to wait for the grace period to // finish. unsafe { self.revoke_internal::<true>() } }}
#[pinned_drop]impl<T> PinnedDrop for Revocable<T> { fn drop(self: Pin<&mut Self>) { // Drop only if the data hasn't been revoked yet (in which case it has already been // dropped). // SAFETY: We are not moving out of `p`, only dropping in place let p = unsafe { self.get_unchecked_mut() }; if *p.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(p.data.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> { // This can't use the `&'a T` type because references that appear in function arguments must // not become dangling during the execution of the function, which can happen if the // `RevocableGuard` is passed as a function argument and then dropped during execution of the // function. 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 } }}