Re: linux-next: manual merge of the char-misc tree with the tip tree

[Boqun Feng]

On Mon, Aug 10, 2026 at 05:58:15PM +0100, Mark Brown wrote:
> Hi all,
> 

Hi Mark,

> Today's linux-next merge of the char-misc tree got a conflict in:
> 
>   rust/kernel/sync/poll.rs
> 
> between commit:
> 
>   47c3367ff096e ("rust: sync: Use safe synchronize_rcu() abstraction in poll")
> 
> from the tip tree and commit:
> 
>   e5e86df8b6661 ("rust: poll: use kfree_rcu() for PollCondVar")
> 
> from the char-misc tree.
> 
> I fixed it up (see below) and can carry the fix as necessary. This
> is now fixed as far as linux-next is concerned, but any non trivial
> conflicts should be mentioned to your upstream maintainer when your tree
> is submitted for merging.  You may also want to consider cooperating
> with the maintainer of the conflicting tree to minimise any particularly
> complex conflicts.
> 

The fix-up looks good to me, thank you!

Regards,
Boqun

> diff --cc rust/kernel/sync/poll.rs
> index 5aa0ce9ba01b7,684dfa242b1aa..0000000000000
> --- a/rust/kernel/sync/poll.rs
> +++ b/rust/kernel/sync/poll.rs
> @@@ -8,13 -9,14 +9,18 @@@ use crate::
>       bindings,
>       fs::File,
>       prelude::*,
>  -    sync::{CondVar, LockClassKey},
>  +    sync::{
>  +        rcu::synchronize_rcu,
>  +        CondVar,
>  +        LockClassKey, //
> -     }, //
> ++    },
> +     types::Opaque, //
> + };
> + use core::{
> +     marker::PhantomData,
> +     mem::ManuallyDrop,
> +     ops::Deref, //
>   };
> - use core::{marker::PhantomData, ops::Deref};
>   
>   /// Creates a [`PollCondVar`] initialiser with the given name and a newly-created lock class.
>   #[macro_export]
> @@@ -103,6 -106,72 +110,70 @@@ impl PinnedDrop for PollCondVar 
>           unsafe { bindings::__wake_up_pollfree(self.inner.wait_queue_head.get()) };
>   
>           // Wait for epoll items to be properly removed.
>  -        //
>  -        // SAFETY: Just an FFI call.
>  -        unsafe { bindings::synchronize_rcu() };
>  +        synchronize_rcu();
>       }
>   }
> + 
> + /// A [`KBox<PollCondVar>`] that uses `kfree_rcu`.
> + ///
> + /// [`KBox<PollCondVar>`]: PollCondVar
> + pub struct PollCondVarBox {
> +     inner: ManuallyDrop<Pin<KBox<PollCondVarBoxInner>>>,
> + }
> + 
> + #[pin_data]
> + #[repr(C)]
> + struct PollCondVarBoxInner {
> +     #[pin]
> +     inner: PollCondVar,
> +     rcu: Opaque<bindings::callback_head>,
> + }
> + 
> + // SAFETY: PollCondVar is Send
> + unsafe impl Send for PollCondVarBoxInner {}
> + // SAFETY: PollCondVar is Sync
> + unsafe impl Sync for PollCondVarBoxInner {}
> + 
> + impl PollCondVarBox {
> +     /// Constructs a new boxed [`PollCondVar`].
> +     pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> Result<Self, AllocError> {
> +         let b = KBox::pin_init(
> +             pin_init!(PollCondVarBoxInner {
> +                 inner <- PollCondVar::new(name, key),
> +                 rcu: Opaque::uninit(),
> +             }),
> +             GFP_KERNEL,
> +         )
> +         .map_err(|_| AllocError)?;
> + 
> +         Ok(PollCondVarBox {
> +             inner: ManuallyDrop::new(b),
> +         })
> +     }
> + }
> + 
> + impl Deref for PollCondVarBox {
> +     type Target = PollCondVar;
> +     fn deref(&self) -> &PollCondVar {
> +         &self.inner.inner
> +     }
> + }
> + 
> + impl Drop for PollCondVarBox {
> +     #[inline]
> +     fn drop(&mut self) {
> +         // SAFETY: ManuallyDrop::take ok because not already taken.
> +         let boxed = unsafe { ManuallyDrop::take(&mut self.inner) };
> + 
> +         // SAFETY: The code below frees the box without calling the actual destructor of the type,
> +         // but it's okay because it re-implements the destructor using `kfree_rcu()` in place of
> +         // `synchronize_rcu()`.
> +         let ptr = KBox::into_raw(unsafe { Pin::into_inner_unchecked(boxed) });
> + 
> +         // SAFETY: The pointer points at a valid `wait_queue_head`.
> +         unsafe { bindings::__wake_up_pollfree((*ptr).inner.inner.wait_queue_head.get()) };
> + 
> +         // SAFETY: This was allocated using `KBox::pin_init`, so it can be freed with `kvfree`.
> +         unsafe { bindings::kvfree_call_rcu((*ptr).rcu.get(), ptr.cast::<ffi::c_void>()) };
> +     }
> + }