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: `LockedBy` serialises the interior mutability it provides, so it is `Sync` as long as the 87 // data it protects is `Send`. 88 unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {} 89 90 impl<T, U> LockedBy<T, U> { 91 /// Constructs a new instance of [`LockedBy`]. 92 /// 93 /// It stores a raw pointer to the owner that is never dereferenced. It is only used to ensure 94 /// that the right owner is being used to access the protected data. If the owner is freed, the 95 /// data becomes inaccessible; if another instance of the owner is allocated *on the same 96 /// memory location*, the data becomes accessible again: none of this affects memory safety 97 /// because in any case at most one thread (or CPU) can access the protected data at a time. 98 pub fn new<B: Backend>(owner: &Lock<U, B>, data: T) -> Self { 99 build_assert!( 100 size_of::<Lock<U, B>>() > 0, 101 "The lock type cannot be a ZST because it may be impossible to distinguish instances" 102 ); 103 Self { 104 owner: owner.data.get(), 105 data: UnsafeCell::new(data), 106 } 107 } 108 } 109 110 impl<T: ?Sized, U> LockedBy<T, U> { 111 /// Returns a reference to the protected data when the caller provides evidence (via a 112 /// reference) that the owner is locked. 113 /// 114 /// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&U` that matches 115 /// the data protected by the lock without actually holding it. 116 /// 117 /// # Panics 118 /// 119 /// Panics if `owner` is different from the data protected by the lock used in 120 /// [`new`](LockedBy::new). 121 pub fn access<'a>(&'a self, owner: &'a U) -> &'a T { 122 build_assert!( 123 size_of::<U>() > 0, 124 "`U` cannot be a ZST because `owner` wouldn't be unique" 125 ); 126 if !ptr::eq(owner, self.owner) { 127 panic!("mismatched owners"); 128 } 129 130 // SAFETY: `owner` is evidence that the owner is locked. 131 unsafe { &*self.data.get() } 132 } 133 134 /// Returns a mutable reference to the protected data when the caller provides evidence (via a 135 /// mutable owner) that the owner is locked mutably. 136 /// 137 /// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&mut U` that 138 /// matches the data protected by the lock without actually holding it. 139 /// 140 /// Showing a mutable reference to the owner is sufficient because we know no other references 141 /// can exist to it. 142 /// 143 /// # Panics 144 /// 145 /// Panics if `owner` is different from the data protected by the lock used in 146 /// [`new`](LockedBy::new). 147 pub fn access_mut<'a>(&'a self, owner: &'a mut U) -> &'a mut T { 148 build_assert!( 149 size_of::<U>() > 0, 150 "`U` cannot be a ZST because `owner` wouldn't be unique" 151 ); 152 if !ptr::eq(owner, self.owner) { 153 panic!("mismatched owners"); 154 } 155 156 // SAFETY: `owner` is evidence that there is only one reference to the owner. 157 unsafe { &mut *self.data.get() } 158 } 159 } 160