mmput_async.rs

 1// SPDX-License-Identifier: GPL-2.0
 2
 3// Copyright (C) 2024 Google LLC.
 4
 5//! Version of `MmWithUser` using `mmput_async`.
 6//!
 7//! This is a separate file from `mm.rs` due to the dependency on `CONFIG_MMU=y`.
 8#![cfg(CONFIG_MMU)]
 9
10use crate::{
11    bindings,
12    mm::MmWithUser,
13    sync::aref::{ARef, AlwaysRefCounted},
14};
15use core::{ops::Deref, ptr::NonNull};
16
17/// A wrapper for the kernel's `struct mm_struct`.
18///
19/// This type is identical to `MmWithUser` except that it uses `mmput_async` when dropping a
20/// refcount. This means that the destructor of `ARef<MmWithUserAsync>` is safe to call in atomic
21/// context.
22///
23/// # Invariants
24///
25/// Values of this type are always refcounted using `mmget`. The value of `mm_users` is non-zero.
26#[repr(transparent)]
27pub struct MmWithUserAsync {
28    mm: MmWithUser,
29}
30
31// SAFETY: It is safe to call `mmput_async` on another thread than where `mmget` was called.
32unsafe impl Send for MmWithUserAsync {}
33// SAFETY: All methods on `MmWithUserAsync` can be called in parallel from several threads.
34unsafe impl Sync for MmWithUserAsync {}
35
36// SAFETY: By the type invariants, this type is always refcounted.
37unsafe impl AlwaysRefCounted for MmWithUserAsync {
38    #[inline]
39    fn inc_ref(&self) {
40        // SAFETY: The pointer is valid since self is a reference.
41        unsafe { bindings::mmget(self.as_raw()) };
42    }
43
44    #[inline]
45    unsafe fn dec_ref(obj: NonNull<Self>) {
46        // SAFETY: The caller is giving up their refcount.
47        unsafe { bindings::mmput_async(obj.cast().as_ptr()) };
48    }
49}
50
51// Make all `MmWithUser` methods available on `MmWithUserAsync`.
52impl Deref for MmWithUserAsync {
53    type Target = MmWithUser;
54
55    #[inline]
56    fn deref(&self) -> &MmWithUser {
57        &self.mm
58    }
59}
60
61impl MmWithUser {
62    /// Use `mmput_async` when dropping this refcount.
63    #[inline]
64    pub fn into_mmput_async(me: ARef<MmWithUser>) -> ARef<MmWithUserAsync> {
65        // SAFETY: The layouts and invariants are compatible.
66        unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
67    }
68}