Re: [PATCH v2] rust: maple_tree: implement Send and Sync for MapleTree

[Joel Fernandes]

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 <joelagnelf@xxxxxxxxxx>
>> ---
>> 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/252a4eef-f4f4-4edf-8154-06cae4ad8518@xxxxxxxxxx/
>>
>>  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/aepRx2jgIKmoRp-r@tardis.local/
> 
> ;-)

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 <boqun@xxxxxxxxxx>
Ok, so I will leave as it is then, thanks for the tag!

--
Joel Fernandes