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

On 9/8/26 11:54 AM, Eliot Courtney wrote:

From: Joel Fernandes <joelagnelf@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@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Boqun Feng <boqun@kernel.org>
[ecourtney: also require T: Sync for the Sync impl]
Signed-off-by: Eliot Courtney <redacted>
---
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.

Acked-by: Danilo Krummrich <dakr@kernel.org>