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 7 use crate::build_assert; 8 use crate::sync::atomic::Atomic; 9 use crate::types::Opaque; 10 11 /// Atomic reference counter. 12 /// 13 /// This type is conceptually an atomic integer, but provides saturation semantics compared to 14 /// normal atomic integers. Values in the negative range when viewed as a signed integer are 15 /// saturation (bad) values. For details about the saturation semantics, please refer to top of 16 /// [`include/linux/refcount.h`](srctree/include/linux/refcount.h). 17 /// 18 /// Wraps the kernel's C `refcount_t`. 19 #[repr(transparent)] 20 pub struct Refcount(Opaque<bindings::refcount_t>); 21 22 impl Refcount { 23 /// Construct a new [`Refcount`] from an initial value. 24 /// 25 /// The initial value should be non-saturated. 26 #[inline] new(value: i32) -> Self27 pub fn new(value: i32) -> Self { 28 build_assert!(value >= 0, "initial value saturated"); 29 // SAFETY: There are no safety requirements for this FFI call. 30 Self(Opaque::new(unsafe { bindings::REFCOUNT_INIT(value) })) 31 } 32 33 #[inline] as_ptr(&self) -> *mut bindings::refcount_t34 fn as_ptr(&self) -> *mut bindings::refcount_t { 35 self.0.get() 36 } 37 38 /// Get the underlying atomic counter that backs the refcount. 39 /// 40 /// NOTE: Usage of this function is discouraged as it can circumvent the protections offered by 41 /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then 42 /// this function can be used. Otherwise consider adding a binding for the required API. 43 #[inline] as_atomic(&self) -> &Atomic<i32>44 pub fn as_atomic(&self) -> &Atomic<i32> { 45 let ptr = self.0.get().cast(); 46 // SAFETY: `refcount_t` is a transparent wrapper of `atomic_t`, which is an atomic 32-bit 47 // integer that is layout-wise compatible with `Atomic<i32>`. All values are valid for 48 // `refcount_t`, despite some of the values being considered saturated and "bad". 49 unsafe { &*ptr } 50 } 51 52 /// Set a refcount's value. 53 #[inline] set(&self, value: i32)54 pub fn set(&self, value: i32) { 55 // SAFETY: `self.as_ptr()` is valid. 56 unsafe { bindings::refcount_set(self.as_ptr(), value) } 57 } 58 59 /// Increment a refcount. 60 /// 61 /// It will saturate if overflows and `WARN`. It will also `WARN` if the refcount is 0, as this 62 /// represents a possible use-after-free condition. 63 /// 64 /// Provides no memory ordering, it is assumed that caller already has a reference on the 65 /// object. 66 #[inline] inc(&self)67 pub fn inc(&self) { 68 // SAFETY: self is valid. 69 unsafe { bindings::refcount_inc(self.as_ptr()) } 70 } 71 72 /// Decrement a refcount. 73 /// 74 /// It will `WARN` on underflow and fail to decrement when saturated. 75 /// 76 /// Provides release memory ordering, such that prior loads and stores are done 77 /// before. 78 #[inline] dec(&self)79 pub fn dec(&self) { 80 // SAFETY: `self.as_ptr()` is valid. 81 unsafe { bindings::refcount_dec(self.as_ptr()) } 82 } 83 84 /// Decrement a refcount and test if it is 0. 85 /// 86 /// It will `WARN` on underflow and fail to decrement when saturated. 87 /// 88 /// Provides release memory ordering, such that prior loads and stores are done 89 /// before, and provides an acquire ordering on success such that memory deallocation 90 /// must come after. 91 /// 92 /// Returns true if the resulting refcount is 0, false otherwise. 93 /// 94 /// # Notes 95 /// 96 /// A common pattern of using `Refcount` is to free memory when the reference count reaches 97 /// zero. This means that the reference to `Refcount` could become invalid after calling this 98 /// function. This is fine as long as the reference to `Refcount` is no longer used when this 99 /// function returns `false`. It is not necessary to use raw pointers in this scenario, see 100 /// <https://github.com/rust-lang/rust/issues/55005>. 101 #[inline] 102 #[must_use = "use `dec` instead if you do not need to test if it is 0"] dec_and_test(&self) -> bool103 pub fn dec_and_test(&self) -> bool { 104 // SAFETY: `self.as_ptr()` is valid. 105 unsafe { bindings::refcount_dec_and_test(self.as_ptr()) } 106 } 107 } 108 109 // SAFETY: `refcount_t` is thread-safe. 110 unsafe impl Send for Refcount {} 111 112 // SAFETY: `refcount_t` is thread-safe. 113 unsafe impl Sync for Refcount {} 114