[PATCH v2] rust: maple_tree: implement Send and Sync for MapleTree
Liam R. Howlett
liam at infradead.org
Tue Sep 8 11:22:25 PDT 2026
On 26/09/08 06:54PM, Eliot Courtney wrote:
> From: Joel Fernandes <joelagnelf at nvidia.com>
>
> The C maple_tree struct contains a *mut c_void, which prevents Rust from
> auto-deriving Send/Sync. Following is an example error message when using
> MapleTree in nova-core's Vmm.
>
> This propagates up through MapleTreeAlloc to Vmm, BarUser, Gpu, and NovaCore,
> causing NovaCore to fail the Send bound required by pci::Driver:
>
> error[E0277]: `*mut c_void` cannot be sent between threads safely
> --> drivers/gpu/nova-core/driver.rs:77:22
> |
> 77 | impl pci::Driver for NovaCore {
> | ^^^^^^^^ `*mut c_void` cannot be sent between threads safely
> |
> = help: within `MapleTreeAlloc<()>`, the trait `Send` is not implemented for `*mut c_void`
> note: required because it appears within the type `kernel::bindings::maple_tree`
> note: required because it appears within the type `Opaque<kernel::bindings::maple_tree>`
> note: required because it appears within the type `MapleTree<()>`
> note: required because it appears within the type `MapleTreeAlloc<()>`
> = note: required for `Box<MapleTreeAlloc<()>, Kmalloc>` to implement `Send`
> note: required because it appears within the type `core::pin::Pin<Box<MapleTreeAlloc<()>, Kmalloc>>`
> note: required because it appears within the type `Vmm`
> note: required because it appears within the type `BarUser`
> note: required because it appears within the type `Gpu`
> note: required because it appears within the type `NovaCore`
> note: required by a bound in `kernel::pci::Driver`
> --> rust/kernel/pci.rs:294:19
>
> Implement Send and Sync for MapleTree. The tree contains no thread-local
> state, and all shared access goes through the internal ma_lock spinlock.
>
> Reviewed-by: Gary Guo <gary at garyguo.net>
> Reviewed-by: Alice Ryhl <aliceryhl at google.com>
> Signed-off-by: Joel Fernandes <joelagnelf at nvidia.com>
> Reviewed-by: Boqun Feng <boqun at kernel.org>
> [ecourtney: also require T: Sync for the Sync impl]
> Signed-off-by: Eliot Courtney <ecourtney at nvidia.com>
Acked-by: Liam R. Howlett (Oracle) <liam at infradead.org>
> ---
> This is a repost of Joel's v3 [1]. The upcoming nova-core memory
> management changes depend on it.
>
> Compared to v3, I've added Boqun's Reviewed-by from the v2 thread [2],
> which v3 didn't pick up.
>
> This is based on drm-rust-next.
>
> [1] https://lore.kernel.org/all/20260511143604.3848176-1-joelagnelf@nvidia.com/
> [2] https://lore.kernel.org/all/aftiZGt3HQe0Bf_x@tardis.local/
> [3] https://lore.kernel.org/all/CAH5fLgh_TQiqKxXsz_DVcQxkCaEw+q0JyTh4e4Tnp28V0JDmYg@mail.gmail.com/
> ---
> Changes in v2:
> - Also require `T: Sync` for the `Sync` impl, so that shared borrows of
> entries (e.g. future RCU readers) are sound [3]. (Alice)
> - Kept Reviewed-by tags since e.g. Boqun also suggested the same `Sync`
> change [4] and Gary was asking around the same kind of thing [5]
> - Link to v1: https://patch.msgid.link/20260908-maple-tree-send-sync-v1-1-43adcd4a16e8@nvidia.com
>
> [4] https://lore.kernel.org/all/aepRx2jgIKmoRp-r@tardis.local/
> [5] https://lore.kernel.org/all/DHYYDQWP4FZ7.3MK090FH4HWID@garyguo.net/
> ---
> rust/kernel/maple_tree.rs | 30 ++++++++++++++++++++++++------
> 1 file changed, 24 insertions(+), 6 deletions(-)
>
> diff --git a/rust/kernel/maple_tree.rs b/rust/kernel/maple_tree.rs
> index 265d6396a78a..7abe41228cb7 100644
> --- a/rust/kernel/maple_tree.rs
> +++ b/rust/kernel/maple_tree.rs
> @@ -16,7 +16,11 @@
> alloc::Flags,
> error::to_result,
> prelude::*,
> - types::{ForeignOwnable, Opaque},
> + types::{
> + ForeignOwnable,
> + NotThreadSafe,
> + Opaque, //
> + },
> };
>
> /// A maple tree optimized for storing non-overlapping ranges.
> @@ -240,7 +244,10 @@ pub fn lock(&self) -> MapleGuard<'_, T> {
> unsafe { bindings::spin_lock(self.ma_lock()) };
>
> // INVARIANT: We just took the spinlock.
> - MapleGuard(self)
> + MapleGuard {
> + tree: self,
> + _not_send: NotThreadSafe,
> + }
> }
>
> #[inline]
> @@ -302,19 +309,30 @@ fn drop(mut self: Pin<&mut Self>) {
> }
> }
>
> +// SAFETY: `MapleTree<T>` is `Send` if `T` is `Send` because `MapleTree` owns its elements.
> +unsafe impl<T: ForeignOwnable + Send> Send for MapleTree<T> {}
> +
> +// SAFETY: `&MapleTree<T>` allows inserting and erasing entries from any thread, so `T: Send` is
> +// required, and shared borrows of entries require `T: Sync`.
> +unsafe impl<T: ForeignOwnable + Send + Sync> Sync for MapleTree<T> {}
> +
> /// A reference to a [`MapleTree`] that owns the inner lock.
> ///
> /// # Invariants
> ///
> /// This guard owns the inner spinlock.
> #[must_use = "if unused, the lock will be immediately unlocked"]
> -pub struct MapleGuard<'tree, T: ForeignOwnable>(&'tree MapleTree<T>);
> +pub struct MapleGuard<'tree, T: ForeignOwnable> {
> + tree: &'tree MapleTree<T>,
> + // A held spinlock must be released on the same CPU that acquired it.
> + _not_send: NotThreadSafe,
> +}
>
> impl<'tree, T: ForeignOwnable> Drop for MapleGuard<'tree, T> {
> #[inline]
> fn drop(&mut self) {
> // SAFETY: By the type invariants, we hold this spinlock.
> - unsafe { bindings::spin_unlock(self.0.ma_lock()) };
> + unsafe { bindings::spin_unlock(self.tree.ma_lock()) };
> }
> }
>
> @@ -323,7 +341,7 @@ impl<'tree, T: ForeignOwnable> MapleGuard<'tree, T> {
> pub fn ma_state(&mut self, first: usize, end: usize) -> MaState<'_, T> {
> // SAFETY: The `MaState` borrows this `MapleGuard`, so it can also borrow the `MapleGuard`s
> // read/write permissions to the maple tree.
> - unsafe { MaState::new_raw(self.0, first, end) }
> + unsafe { MaState::new_raw(self.tree, first, end) }
> }
>
> /// Load the value at the given index.
> @@ -375,7 +393,7 @@ pub fn ma_state(&mut self, first: usize, end: usize) -> MaState<'_, T> {
> #[inline]
> pub fn load(&mut self, index: usize) -> Option<T::BorrowedMut<'_>> {
> // SAFETY: `self.tree` contains a valid maple tree.
> - let ret = unsafe { bindings::mtree_load(self.0.tree.get(), index) };
> + let ret = unsafe { bindings::mtree_load(self.tree.tree.get(), index) };
> if ret.is_null() {
> return None;
> }
>
> ---
> base-commit: e6a2c988ed96a5a3af51ed97ecba980521bf2fc0
> change-id: 20260907-maple-tree-send-sync-a1479d1ab302
>
> Best regards,
> --
> Eliot Courtney <ecourtney at nvidia.com>
>
>
> --
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