1 // SPDX-License-Identifier: GPL-2.0
2
3 //! A wrapper for data protected by a lock that does not wrap it.
4
5 use super::{lock::Backend, lock::Lock};
6 use crate::build_assert;
7 use core::{cell::UnsafeCell, mem::size_of, ptr};
8
9 /// Allows access to some data to be serialised by a lock that does not wrap it.
10 ///
11 /// In most cases, data protected by a lock is wrapped by the appropriate lock type, e.g.,
12 /// [`Mutex`] or [`SpinLock`]. [`LockedBy`] is meant for cases when this is not possible.
13 /// For example, if a container has a lock and some data in the contained elements needs
14 /// to be protected by the same lock.
15 ///
16 /// [`LockedBy`] wraps the data in lieu of another locking primitive, and only allows access to it
17 /// when the caller shows evidence that the 'external' lock is locked. It panics if the evidence
18 /// refers to the wrong instance of the lock.
19 ///
20 /// [`Mutex`]: super::Mutex
21 /// [`SpinLock`]: super::SpinLock
22 ///
23 /// # Examples
24 ///
25 /// The following is an example for illustrative purposes: `InnerDirectory::bytes_used` is an
26 /// aggregate of all `InnerFile::bytes_used` and must be kept consistent; so we wrap `InnerFile` in
27 /// a `LockedBy` so that it shares a lock with `InnerDirectory`. This allows us to enforce at
28 /// compile-time that access to `InnerFile` is only granted when an `InnerDirectory` is also
29 /// locked; we enforce at run time that the right `InnerDirectory` is locked.
30 ///
31 /// ```
32 /// use kernel::sync::{LockedBy, Mutex};
33 ///
34 /// struct InnerFile {
35 /// bytes_used: u64,
36 /// }
37 ///
38 /// struct File {
39 /// _ino: u32,
40 /// inner: LockedBy<InnerFile, InnerDirectory>,
41 /// }
42 ///
43 /// struct InnerDirectory {
44 /// /// The sum of the bytes used by all files.
45 /// bytes_used: u64,
46 /// _files: Vec<File>,
47 /// }
48 ///
49 /// struct Directory {
50 /// _ino: u32,
51 /// inner: Mutex<InnerDirectory>,
52 /// }
53 ///
54 /// /// Prints `bytes_used` from both the directory and file.
55 /// fn print_bytes_used(dir: &Directory, file: &File) {
56 /// let guard = dir.inner.lock();
57 /// let inner_file = file.inner.access(&guard);
58 /// pr_info!("{} {}", guard.bytes_used, inner_file.bytes_used);
59 /// }
60 ///
61 /// /// Increments `bytes_used` for both the directory and file.
62 /// fn inc_bytes_used(dir: &Directory, file: &File) {
63 /// let mut guard = dir.inner.lock();
64 /// guard.bytes_used += 10;
65 ///
66 /// let file_inner = file.inner.access_mut(&mut guard);
67 /// file_inner.bytes_used += 10;
68 /// }
69 ///
70 /// /// Creates a new file.
71 /// fn new_file(ino: u32, dir: &Directory) -> File {
72 /// File {
73 /// _ino: ino,
74 /// inner: LockedBy::new(&dir.inner, InnerFile { bytes_used: 0 }),
75 /// }
76 /// }
77 /// ```
78 pub struct LockedBy<T: ?Sized, U: ?Sized> {
79 owner: *const U,
80 data: UnsafeCell<T>,
81 }
82
83 // SAFETY: `LockedBy` can be transferred across thread boundaries iff the data it protects can.
84 unsafe impl<T: ?Sized + Send, U: ?Sized> Send for LockedBy<T, U> {}
85
86 // SAFETY: If `T` is not `Sync`, then parallel shared access to this `LockedBy` allows you to use
87 // `access_mut` to hand out `&mut T` on one thread at the time. The requirement that `T: Send` is
88 // sufficient to allow that.
89 //
90 // If `T` is `Sync`, then the `access` method also becomes available, which allows you to obtain
91 // several `&T` from several threads at once. However, this is okay as `T` is `Sync`.
92 unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {}
93
94 impl<T, U> LockedBy<T, U> {
95 /// Constructs a new instance of [`LockedBy`].
96 ///
97 /// It stores a raw pointer to the owner that is never dereferenced. It is only used to ensure
98 /// that the right owner is being used to access the protected data. If the owner is freed, the
99 /// data becomes inaccessible; if another instance of the owner is allocated *on the same
100 /// memory location*, the data becomes accessible again: none of this affects memory safety
101 /// because in any case at most one thread (or CPU) can access the protected data at a time.
102 pub fn new<B: Backend>(owner: &Lock<U, B>, data: T) -> Self {
103 build_assert!(
104 size_of::<Lock<U, B>>() > 0,
105 "The lock type cannot be a ZST because it may be impossible to distinguish instances"
106 );
107 Self {
108 owner: owner.data.get(),
109 data: UnsafeCell::new(data),
110 }
111 }
112 }
113
114 impl<T: ?Sized, U> LockedBy<T, U> {
115 /// Returns a reference to the protected data when the caller provides evidence (via a
116 /// reference) that the owner is locked.
117 ///
118 /// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&U` that matches
119 /// the data protected by the lock without actually holding it.
120 ///
121 /// # Panics
122 ///
123 /// Panics if `owner` is different from the data protected by the lock used in
124 /// [`new`](LockedBy::new).
125 pub fn access<'a>(&'a self, owner: &'a U) -> &'a T
126 where
127 T: Sync,
128 {
129 build_assert!(
130 size_of::<U>() > 0,
131 "`U` cannot be a ZST because `owner` wouldn't be unique"
132 );
133 if !ptr::eq(owner, self.owner) {
134 panic!("mismatched owners");
135 }
136
137 // SAFETY: `owner` is evidence that there are only shared references to the owner for the
138 // duration of 'a, so it's not possible to use `Self::access_mut` to obtain a mutable
139 // reference to the inner value that aliases with this shared reference. The type is `Sync`
140 // so there are no other requirements.
141 unsafe { &*self.data.get() }
142 }
143
144 /// Returns a mutable reference to the protected data when the caller provides evidence (via a
145 /// mutable owner) that the owner is locked mutably.
146 ///
147 /// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&mut U` that
148 /// matches the data protected by the lock without actually holding it.
149 ///
150 /// Showing a mutable reference to the owner is sufficient because we know no other references
151 /// can exist to it.
152 ///
153 /// # Panics
154 ///
155 /// Panics if `owner` is different from the data protected by the lock used in
156 /// [`new`](LockedBy::new).
157 pub fn access_mut<'a>(&'a self, owner: &'a mut U) -> &'a mut T {
158 build_assert!(
159 size_of::<U>() > 0,
160 "`U` cannot be a ZST because `owner` wouldn't be unique"
161 );
162 if !ptr::eq(owner, self.owner) {
163 panic!("mismatched owners");
164 }
165
166 // SAFETY: `owner` is evidence that there is only one reference to the owner.
167 unsafe { &mut *self.data.get() }
168 }
169 }
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