On 5/6/2026 11:46 AM, Boqun Feng wrote:
> On Wed, May 06, 2026 at 09:51:22AM -0400, Joel Fernandes wrote:
>> 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.
>>
>> Signed-off-by: Joel Fernandes <[email protected]>
>> ---
>> RFC->v2: Just adjusted a few comments as suggested by Gary.
>>
>> Sending this separately as discussed in the nova mm patch series that needs
>> it:
>> https://lore.kernel.org/all/[email protected]/
>>
>> rust/kernel/maple_tree.rs | 29 +++++++++++++++++++++++------
>> 1 file changed, 23 insertions(+), 6 deletions(-)
>>
>> diff --git a/rust/kernel/maple_tree.rs b/rust/kernel/maple_tree.rs
>> index 265d6396a78a..2400c905270d 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 +309,29 @@ 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>` never hands out `&T`; all entry access is
>> serialized
>> +// by `ma_lock` or `&mut Guard`, so `T: Send` suffices (`T: Sync` not
>> required).
>> +unsafe impl<T: ForeignOwnable + Send> Sync for MapleTree<T> {}
>
> I think you missed this:
>
> https://lore.kernel.org/rust-for-linux/[email protected]/
>
> ;-)
Oops, sorry about that.
>
> But on a second thought, should that (we have methods of `&MapleTree<T>`
> -> `&T`) happen, we could always guard those methods with T: Sync. Maybe
> it's worth to keep a note on that but, anyway
>
> Reviewed-by: Boqun Feng <[email protected]>
Ok, so I will leave as it is then, thanks for the tag!
--
Joel Fernandes
