Adds an MmWithUserAsync type that uses mmput_async when dropped but is
otherwise identical to MmWithUser. This has to be done using a separate
type because the thing we are changing is the destructor.
Rust Binder needs this to avoid a certain deadlock. See commit
9a9ab0d96362 ("binder: fix race between mmput() and do_exit()") for
details. It's also needed in the shrinker to avoid cleaning up the mm in
the shrinker's context.
Acked-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> (for mm bits)
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
rust/kernel/mm.rs | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)
diff --git a/rust/kernel/mm.rs b/rust/kernel/mm.rs
index 425b73a9dfe6..50f4861ae4b9 100644
--- a/rust/kernel/mm.rs
+++ b/rust/kernel/mm.rs
@@ -98,6 +98,48 @@ fn deref(&self) -> &Mm {
}
}
+/// A wrapper for the kernel's `struct mm_struct`.
+///
+/// This type is identical to `MmWithUser` except that it uses `mmput_async` when dropping a
+/// refcount. This means that the destructor of `ARef<MmWithUserAsync>` is safe to call in atomic
+/// context.
+///
+/// # Invariants
+///
+/// Values of this type are always refcounted using `mmget`. The value of `mm_users` is non-zero.
+#[repr(transparent)]
+pub struct MmWithUserAsync {
+ mm: MmWithUser,
+}
+
+// SAFETY: It is safe to call `mmput_async` on another thread than where `mmget` was called.
+unsafe impl Send for MmWithUserAsync {}
+// SAFETY: All methods on `MmWithUserAsync` can be called in parallel from several threads.
+unsafe impl Sync for MmWithUserAsync {}
+
+// SAFETY: By the type invariants, this type is always refcounted.
+unsafe impl AlwaysRefCounted for MmWithUserAsync {
+ fn inc_ref(&self) {
+ // SAFETY: The pointer is valid since self is a reference.
+ unsafe { bindings::mmget(self.as_raw()) };
+ }
+
+ unsafe fn dec_ref(obj: NonNull<Self>) {
+ // SAFETY: The caller is giving up their refcount.
+ unsafe { bindings::mmput_async(obj.cast().as_ptr()) };
+ }
+}
+
+// Make all `MmWithUser` methods available on `MmWithUserAsync`.
+impl Deref for MmWithUserAsync {
+ type Target = MmWithUser;
+
+ #[inline]
+ fn deref(&self) -> &MmWithUser {
+ &self.mm
+ }
+}
+
// These methods are safe to call even if `mm_users` is zero.
impl Mm {
/// Call `mmgrab` on `current.mm`.
@@ -171,6 +213,13 @@ pub unsafe fn from_raw<'a>(ptr: *const bindings::mm_struct) -> &'a MmWithUser {
unsafe { &*ptr.cast() }
}
+ /// Use `mmput_async` when dropping this refcount.
+ #[inline]
+ pub fn into_mmput_async(me: ARef<MmWithUser>) -> ARef<MmWithUserAsync> {
+ // SAFETY: The layouts and invariants are compatible.
+ unsafe { ARef::from_raw(ARef::into_raw(me).cast()) }
+ }
+
/// Attempt to access a vma using the vma read lock.
///
/// This is an optimistic trylock operation, so it may fail if there is contention. In that
--
2.47.1.613.gc27f4b7a9f-goog
