revocable.rs

  1// SPDX-License-Identifier: GPL-2.0
  2
  3//! Revocable objects.
  4//!
  5//! The [`Revocable`] type wraps other types and allows access to them to be revoked. The existence
  6//! of a [`RevocableGuard`] ensures that objects remain valid.
  7
  8use pin_init::Wrapper;
  9
 10use crate::{bindings, prelude::*, sync::rcu, types::Opaque};
 11use core::{
 12    marker::PhantomData,
 13    ops::Deref,
 14    ptr::drop_in_place,
 15    sync::atomic::{AtomicBool, Ordering},
 16};
 17
 18/// An object that can become inaccessible at runtime.
 19///
 20/// Once access is revoked and all concurrent users complete (i.e., all existing instances of
 21/// [`RevocableGuard`] are dropped), the wrapped object is also dropped.
 22///
 23/// # Examples
 24///
 25/// ```
 26/// # use kernel::revocable::Revocable;
 27///
 28/// struct Example {
 29///     a: u32,
 30///     b: u32,
 31/// }
 32///
 33/// fn add_two(v: &Revocable<Example>) -> Option<u32> {
 34///     let guard = v.try_access()?;
 35///     Some(guard.a + guard.b)
 36/// }
 37///
 38/// let v = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap();
 39/// assert_eq!(add_two(&v), Some(30));
 40/// v.revoke();
 41/// assert_eq!(add_two(&v), None);
 42/// ```
 43///
 44/// Sample example as above, but explicitly using the rcu read side lock.
 45///
 46/// ```
 47/// # use kernel::revocable::Revocable;
 48/// use kernel::sync::rcu;
 49///
 50/// struct Example {
 51///     a: u32,
 52///     b: u32,
 53/// }
 54///
 55/// fn add_two(v: &Revocable<Example>) -> Option<u32> {
 56///     let guard = rcu::read_lock();
 57///     let e = v.try_access_with_guard(&guard)?;
 58///     Some(e.a + e.b)
 59/// }
 60///
 61/// let v = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap();
 62/// assert_eq!(add_two(&v), Some(30));
 63/// v.revoke();
 64/// assert_eq!(add_two(&v), None);
 65/// ```
 66#[pin_data(PinnedDrop)]
 67pub struct Revocable<T> {
 68    is_available: AtomicBool,
 69    #[pin]
 70    data: Opaque<T>,
 71}
 72
 73// SAFETY: `Revocable` is `Send` if the wrapped object is also `Send`. This is because while the
 74// functionality exposed by `Revocable` can be accessed from any thread/CPU, it is possible that
 75// this isn't supported by the wrapped object.
 76unsafe impl<T: Send> Send for Revocable<T> {}
 77
 78// SAFETY: `Revocable` is `Sync` if the wrapped object is both `Send` and `Sync`. We require `Send`
 79// from the wrapped object as well because  of `Revocable::revoke`, which can trigger the `Drop`
 80// implementation of the wrapped object from an arbitrary thread.
 81unsafe impl<T: Sync + Send> Sync for Revocable<T> {}
 82
 83impl<T> Revocable<T> {
 84    /// Creates a new revocable instance of the given data.
 85    pub fn new<E>(data: impl PinInit<T, E>) -> impl PinInit<Self, E> {
 86        try_pin_init!(Self {
 87            is_available: AtomicBool::new(true),
 88            data <- Opaque::pin_init(data),
 89        }? E)
 90    }
 91
 92    /// Tries to access the revocable wrapped object.
 93    ///
 94    /// Returns `None` if the object has been revoked and is therefore no longer accessible.
 95    ///
 96    /// Returns a guard that gives access to the object otherwise; the object is guaranteed to
 97    /// remain accessible while the guard is alive. In such cases, callers are not allowed to sleep
 98    /// because another CPU may be waiting to complete the revocation of this object.
 99    pub fn try_access(&self) -> Option<RevocableGuard<'_, T>> {
100        let guard = rcu::read_lock();
101        if self.is_available.load(Ordering::Relaxed) {
102            // Since `self.is_available` is true, data is initialised and has to remain valid
103            // because the RCU read side lock prevents it from being dropped.
104            Some(RevocableGuard::new(self.data.get(), guard))
105        } else {
106            None
107        }
108    }
109
110    /// Tries to access the revocable wrapped object.
111    ///
112    /// Returns `None` if the object has been revoked and is therefore no longer accessible.
113    ///
114    /// Returns a shared reference to the object otherwise; the object is guaranteed to
115    /// remain accessible while the rcu read side guard is alive. In such cases, callers are not
116    /// allowed to sleep because another CPU may be waiting to complete the revocation of this
117    /// object.
118    pub fn try_access_with_guard<'a>(&'a self, _guard: &'a rcu::Guard) -> Option<&'a T> {
119        if self.is_available.load(Ordering::Relaxed) {
120            // SAFETY: Since `self.is_available` is true, data is initialised and has to remain
121            // valid because the RCU read side lock prevents it from being dropped.
122            Some(unsafe { &*self.data.get() })
123        } else {
124            None
125        }
126    }
127
128    /// Tries to access the wrapped object and run a closure on it while the guard is held.
129    ///
130    /// This is a convenience method to run short non-sleepable code blocks while ensuring the
131    /// guard is dropped afterwards. [`Self::try_access`] carries the risk that the caller will
132    /// forget to explicitly drop that returned guard before calling sleepable code; this method
133    /// adds an extra safety to make sure it doesn't happen.
134    ///
135    /// Returns [`None`] if the object has been revoked and is therefore no longer accessible, or
136    /// the result of the closure wrapped in [`Some`]. If the closure returns a [`Result`] then the
137    /// return type becomes `Option<Result<>>`, which can be inconvenient. Users are encouraged to
138    /// define their own macro that turns the [`Option`] into a proper error code and flattens the
139    /// inner result into it if it makes sense within their subsystem.
140    pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
141        self.try_access().map(|t| f(&*t))
142    }
143
144    /// Directly access the revocable wrapped object.
145    ///
146    /// # Safety
147    ///
148    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked
149    /// as long as the returned `&T` lives.
150    pub unsafe fn access(&self) -> &T {
151        // SAFETY: By the safety requirement of this function it is guaranteed that
152        // `self.data.get()` is a valid pointer to an instance of `T`.
153        unsafe { &*self.data.get() }
154    }
155
156    /// # Safety
157    ///
158    /// Callers must ensure that there are no more concurrent users of the revocable object.
159    unsafe fn revoke_internal<const SYNC: bool>(&self) -> bool {
160        let revoke = self.is_available.swap(false, Ordering::Relaxed);
161
162        if revoke {
163            if SYNC {
164                // SAFETY: Just an FFI call, there are no further requirements.
165                unsafe { bindings::synchronize_rcu() };
166            }
167
168            // SAFETY: We know `self.data` is valid because only one CPU can succeed the
169            // `compare_exchange` above that takes `is_available` from `true` to `false`.
170            unsafe { drop_in_place(self.data.get()) };
171        }
172
173        revoke
174    }
175
176    /// Revokes access to and drops the wrapped object.
177    ///
178    /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`],
179    /// expecting that there are no concurrent users of the object.
180    ///
181    /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked
182    /// already.
183    ///
184    /// # Safety
185    ///
186    /// Callers must ensure that there are no more concurrent users of the revocable object.
187    pub unsafe fn revoke_nosync(&self) -> bool {
188        // SAFETY: By the safety requirement of this function, the caller ensures that nobody is
189        // accessing the data anymore and hence we don't have to wait for the grace period to
190        // finish.
191        unsafe { self.revoke_internal::<false>() }
192    }
193
194    /// Revokes access to and drops the wrapped object.
195    ///
196    /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`].
197    ///
198    /// If there are concurrent users of the object (i.e., ones that called
199    /// [`Revocable::try_access`] beforehand and still haven't dropped the returned guard), this
200    /// function waits for the concurrent access to complete before dropping the wrapped object.
201    ///
202    /// Returns `true` if `&self` has been revoked with this call, `false` if it was revoked
203    /// already.
204    pub fn revoke(&self) -> bool {
205        // SAFETY: By passing `true` we ask `revoke_internal` to wait for the grace period to
206        // finish.
207        unsafe { self.revoke_internal::<true>() }
208    }
209}
210
211#[pinned_drop]
212impl<T> PinnedDrop for Revocable<T> {
213    fn drop(self: Pin<&mut Self>) {
214        // Drop only if the data hasn't been revoked yet (in which case it has already been
215        // dropped).
216        // SAFETY: We are not moving out of `p`, only dropping in place
217        let p = unsafe { self.get_unchecked_mut() };
218        if *p.is_available.get_mut() {
219            // SAFETY: We know `self.data` is valid because no other CPU has changed
220            // `is_available` to `false` yet, and no other CPU can do it anymore because this CPU
221            // holds the only reference (mutable) to `self` now.
222            unsafe { drop_in_place(p.data.get()) };
223        }
224    }
225}
226
227/// A guard that allows access to a revocable object and keeps it alive.
228///
229/// CPUs may not sleep while holding on to [`RevocableGuard`] because it's in atomic context
230/// holding the RCU read-side lock.
231///
232/// # Invariants
233///
234/// The RCU read-side lock is held while the guard is alive.
235pub struct RevocableGuard<'a, T> {
236    // This can't use the `&'a T` type because references that appear in function arguments must
237    // not become dangling during the execution of the function, which can happen if the
238    // `RevocableGuard` is passed as a function argument and then dropped during execution of the
239    // function.
240    data_ref: *const T,
241    _rcu_guard: rcu::Guard,
242    _p: PhantomData<&'a ()>,
243}
244
245impl<T> RevocableGuard<'_, T> {
246    fn new(data_ref: *const T, rcu_guard: rcu::Guard) -> Self {
247        Self {
248            data_ref,
249            _rcu_guard: rcu_guard,
250            _p: PhantomData,
251        }
252    }
253}
254
255impl<T> Deref for RevocableGuard<'_, T> {
256    type Target = T;
257
258    fn deref(&self) -> &Self::Target {
259        // SAFETY: By the type invariants, we hold the rcu read-side lock, so the object is
260        // guaranteed to remain valid.
261        unsafe { &*self.data_ref }
262    }
263}