From: Wedson Almeida Filho <wedso...@gmail.com>

Revocable allows access to objects to be safely revoked at run time.

This is useful, for example, for resources allocated during device probe;
when the device is removed, the driver should stop accessing the device
resources even if another state is kept in memory due to existing
references (i.e., device context data is ref-counted and has a non-zero
refcount after removal of the device).

Signed-off-by: Wedson Almeida Filho <wedso...@gmail.com>
Co-developed-by: Danilo Krummrich <d...@kernel.org>
Signed-off-by: Danilo Krummrich <d...@kernel.org>
---
 rust/kernel/lib.rs       |   1 +
 rust/kernel/revocable.rs | 219 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 220 insertions(+)
 create mode 100644 rust/kernel/revocable.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 66149ac5c0c9..5702ce32ec8e 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -60,6 +60,7 @@
 pub mod prelude;
 pub mod print;
 pub mod rbtree;
+pub mod revocable;
 pub mod security;
 pub mod seq_file;
 pub mod sizes;
diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
new file mode 100644
index 000000000000..1e5a9d25c21b
--- /dev/null
+++ b/rust/kernel/revocable.rs
@@ -0,0 +1,219 @@
+// 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, prelude::*, sync::rcu, types::Opaque};
+use core::{
+    marker::PhantomData,
+    ops::Deref,
+    ptr::drop_in_place,
+    sync::atomic::{AtomicBool, 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 = 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: AtomicBool,
+    #[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(data: impl PinInit<T>) -> impl PinInit<Self> {
+        pin_init!(Self {
+            is_available: AtomicBool::new(true),
+            data <- Opaque::pin_init(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) {
+            // 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(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.get() })
+        } else {
+            None
+        }
+    }
+
+    /// # Safety
+    ///
+    /// Callers must ensure that there are no more concurrent users of the 
revocable object.
+    unsafe fn revoke_internal<const SYNC: bool>(&self) {
+        if self.is_available.swap(false, Ordering::Relaxed) {
+            if SYNC {
+                // 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.get()) };
+        }
+    }
+
+    /// 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.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that there are no more concurrent users of the 
revocable object.
+    pub unsafe fn revoke_nosync(&self) {
+        // 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.
+    pub fn revoke(&self) {
+        // 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> {
+    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 }
+    }
+}
-- 
2.47.1


Reply via email to