Re: [PATCH] rust: mutex: fix __mutex_init() usage in case of PREEMPT_RT

[Gary Guo <gary@garyguo.net>]

On Mon, 16 Sep 2024 09:37:52 +0200
Dirk Behme <dirk.behme@de.bosch.com> wrote:

> In case CONFIG_PREEMPT_RT is enabled __mutex_init() becomes a macro
> instead of an extern function (simplified from
> include/linux/mutex.h):
> 
> extern void __mutex_init(struct mutex *lock, const char *name,
> 			 struct lock_class_key *key);
> do {							\
> 	rt_mutex_base_init(&(mutex)->rtmutex);		\
> 	__mutex_rt_init((mutex), name, key);		\
> } while (0)
> 
> The macro isn't resolved by bindgen, then. What results in a build
> error:
> 
> error[E0425]: cannot find function `__mutex_init` in crate `bindings`
>      --> rust/kernel/sync/lock/mutex.rs:104:28  
>       |
> 104   |           unsafe { bindings::__mutex_init(ptr, name, key) }
>       |                              ^^^^^^^^^^^^ help: a function with a similar name exists: `__mutex_rt_init`
>       |
>      ::: rust/bindings/bindings_generated.rs:23722:5
>       |
> 23722 | /     pub fn __mutex_rt_init(
> 23723 | |         lock: *mut mutex,
> 23724 | |         name: *const core::ffi::c_char,
> 23725 | |         key: *mut lock_class_key,
> 23726 | |     );
>       | |_____- similarly named function `__mutex_rt_init` defined here
> 
> Fix this by adding a helper.
> 
> As explained by Gary Guo in [1] no #ifdef CONFIG_PREEMPT_RT
> is needed here as rust/bindings/lib.rs prefers externed function to
> helpers if an externed function exists.
> 
> Reported-by: Conor Dooley <conor@kernel.org>
> Link: https://lore.kernel.org/rust-for-linux/20240913-shack-estate-b376a65921b1@spud/
> Link: https://lore.kernel.org/rust-for-linux/20240915123626.1a170103.gary@garyguo.net/ [1]
> Fixes: 6d20d629c6d8 ("rust: lock: introduce `Mutex`")
> Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>

Hi Dirk,

Reviewed-by: Gary Guo <gary@garyguo.net>

Thanks for sending the patch.

Best,
Gary

> ---
> 
> This is against rust-next to use the new helpers layout.
> Due to the helpers layout change I'm thinking it is not worth
> the effort to backport to older kernel versions, so I intentionally
> did not include -stable.
> 
>  rust/helpers/mutex.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/rust/helpers/mutex.c b/rust/helpers/mutex.c
> index 200db7e6279f0..a17ca8cdb50ca 100644
> --- a/rust/helpers/mutex.c
> +++ b/rust/helpers/mutex.c
> @@ -7,3 +7,9 @@ void rust_helper_mutex_lock(struct mutex *lock)
>  {
>  	mutex_lock(lock);
>  }
> +
> +void rust_helper___mutex_init(struct mutex *mutex, const char *name,
> +			      struct lock_class_key *key)
> +{
> +	__mutex_init(mutex, name, key);
> +}