Re: linux-next: manual merge of the slab tree with the rcu tree

[Miguel Ojeda]

On Thu, 30 Jul 2026 15:38:48 +0100 Mark Brown <broonie@xxxxxxxxxx> wrote:
>
> Today's linux-next merge of the slab tree got a conflict in:
>
>   mm/slab_common.c
>
> between commit:
>
>   736084507faa3 ("rcu: Rename struct rcu_gp_oldstate to rcu_gp_seq")
>
> from the rcu tree and commit:
>
>   bdd0cc9f0ffc3 ("mm/slab: introduce struct kvfree_rcu_head for kvfree_rcu batching")
>
> from the slab tree.

The slab one semantically conflicts as well with commit:

  e5e86df8b666 ("rust: poll: use kfree_rcu() for PollCondVar")

from the char-misc tree:

    error[E0308]: mismatched types
         --> rust/kernel/sync/poll.rs:175:44
          |
      175 |         unsafe { bindings::kvfree_call_rcu((*ptr).rcu.get(), ptr.cast::<ffi::c_void>()) };
          |                  ------------------------- ^^^^^^^^^^^^^^^^ expected `*mut kvfree_rcu_head`, found `*mut callback_head`
          |                  |
          |                  arguments to this function are incorrect
          |
          = note: expected raw pointer `*mut bindings::kvfree_rcu_head`
                     found raw pointer `*mut bindings::callback_head`
    note: function defined here
         --> rust/bindings/bindings_generated.rs:90407:12
          |
    90407 |     pub fn kvfree_call_rcu(head: *mut kvfree_rcu_head, ptr: *mut ffi::c_void);
          |            ^^^^^^^^^^^^^^^

The C API was not meant to have a visible effect, but we don't use the
`kvfree_rcu()` macro, so we notice the change. We can add a similar cast
to the C side:

    diff --git a/rust/kernel/sync/poll.rs b/rust/kernel/sync/poll.rs
    index 684dfa242b1a..79d7e08215b0 100644
    --- a/rust/kernel/sync/poll.rs
    +++ b/rust/kernel/sync/poll.rs
    @@ -172,6 +172,6 @@ fn drop(&mut self) {
             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>()) };
    +        unsafe { bindings::kvfree_call_rcu((*ptr).rcu.get().cast(), ptr.cast::<ffi::c_void>()) };
         }
     }

Clippy doesn't complain about the "unneeded" `cast()` under
`CONFIG_KVFREE_RCU_BATCHED=n`, so it should be fine.

Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Cc: Boqun Feng <boqun@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
Cc: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Cheers,
Miguel