kernel/sync/
refcount.rs

1// SPDX-License-Identifier: GPL-2.0
2
3//! Atomic reference counting.
4//!
5//! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h)
6
7use crate::{
8    build_assert::build_assert,
9    sync::atomic::Atomic,
10    types::Opaque, //
11};
12
13/// Atomic reference counter.
14///
15/// This type is conceptually an atomic integer, but provides saturation semantics compared to
16/// normal atomic integers. Values in the negative range when viewed as a signed integer are
17/// saturation (bad) values. For details about the saturation semantics, please refer to top of
18/// [`include/linux/refcount.h`](srctree/include/linux/refcount.h).
19///
20/// Wraps the kernel's C `refcount_t`.
21#[repr(transparent)]
22pub struct Refcount(Opaque<bindings::refcount_t>);
23
24impl Refcount {
25    /// Construct a new [`Refcount`] from an initial value.
26    ///
27    /// The initial value should be non-saturated.
28    // Always inline to optimize out error path of `build_assert`.
29    #[inline(always)]
30    pub fn new(value: i32) -> Self {
31        build_assert!(value >= 0, "initial value saturated");
32        // SAFETY: There are no safety requirements for this FFI call.
33        Self(Opaque::new(unsafe { bindings::REFCOUNT_INIT(value) }))
34    }
35
36    #[inline]
37    fn as_ptr(&self) -> *mut bindings::refcount_t {
38        self.0.get()
39    }
40
41    /// Get the underlying atomic counter that backs the refcount.
42    ///
43    /// NOTE: Usage of this function is discouraged as it can circumvent the protections offered by
44    /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then
45    /// this function can be used. Otherwise consider adding a binding for the required API.
46    #[inline]
47    pub fn as_atomic(&self) -> &Atomic<i32> {
48        let ptr = self.0.get().cast();
49        // SAFETY: `refcount_t` is a transparent wrapper of `atomic_t`, which is an atomic 32-bit
50        // integer that is layout-wise compatible with `Atomic<i32>`. All values are valid for
51        // `refcount_t`, despite some of the values being considered saturated and "bad".
52        unsafe { &*ptr }
53    }
54
55    /// Set a refcount's value.
56    #[inline]
57    pub fn set(&self, value: i32) {
58        // SAFETY: `self.as_ptr()` is valid.
59        unsafe { bindings::refcount_set(self.as_ptr(), value) }
60    }
61
62    /// Increment a refcount.
63    ///
64    /// It will saturate if overflows and `WARN`. It will also `WARN` if the refcount is 0, as this
65    /// represents a possible use-after-free condition.
66    ///
67    /// Provides no memory ordering, it is assumed that caller already has a reference on the
68    /// object.
69    #[inline]
70    pub fn inc(&self) {
71        // SAFETY: self is valid.
72        unsafe { bindings::refcount_inc(self.as_ptr()) }
73    }
74
75    /// Decrement a refcount.
76    ///
77    /// It will `WARN` on underflow and fail to decrement when saturated.
78    ///
79    /// Provides release memory ordering, such that prior loads and stores are done
80    /// before.
81    #[inline]
82    pub fn dec(&self) {
83        // SAFETY: `self.as_ptr()` is valid.
84        unsafe { bindings::refcount_dec(self.as_ptr()) }
85    }
86
87    /// Decrement a refcount and test if it is 0.
88    ///
89    /// It will `WARN` on underflow and fail to decrement when saturated.
90    ///
91    /// Provides release memory ordering, such that prior loads and stores are done
92    /// before, and provides an acquire ordering on success such that memory deallocation
93    /// must come after.
94    ///
95    /// Returns true if the resulting refcount is 0, false otherwise.
96    ///
97    /// # Notes
98    ///
99    /// A common pattern of using `Refcount` is to free memory when the reference count reaches
100    /// zero. This means that the reference to `Refcount` could become invalid after calling this
101    /// function. This is fine as long as the reference to `Refcount` is no longer used when this
102    /// function returns `false`. It is not necessary to use raw pointers in this scenario, see
103    /// <https://github.com/rust-lang/rust/issues/55005>.
104    #[inline]
105    #[must_use = "use `dec` instead if you do not need to test if it is 0"]
106    pub fn dec_and_test(&self) -> bool {
107        // SAFETY: `self.as_ptr()` is valid.
108        unsafe { bindings::refcount_dec_and_test(self.as_ptr()) }
109    }
110}
111
112// SAFETY: `refcount_t` is thread-safe.
113unsafe impl Send for Refcount {}
114
115// SAFETY: `refcount_t` is thread-safe.
116unsafe impl Sync for Refcount {}
kernel/sync/refcount.rs

kernel/sync/
refcount.rs
