Re: [PATCH 1/1] hung_task: fix warnings by enforcing alignment on lock structures

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi Lence,

On Sat, 23 Aug 2025 15:40:48 +0800
Lance Yang <lance.yang@linux.dev> wrote:

> From: Lance Yang <lance.yang@linux.dev>
> 
> The blocker tracking mechanism assumes that lock pointers are at least
> 4-byte aligned to use their lower bits for type encoding.
> 
> However, as reported by Geert Uytterhoeven, some architectures like m68k
> only guarantee 2-byte alignment of 32-bit values. This breaks the
> assumption and causes two related WARN_ON_ONCE checks to trigger.
> 
> To fix this, enforce a minimum of 4-byte alignment on the core lock
> structures supported by the blocker tracking mechanism. This ensures the
> algorithm's alignment assumption now holds true on all architectures.
> 
> This patch adds __aligned(4) to the definitions of "struct mutex",
> "struct semaphore", and "struct rw_semaphore", resolving the warnings.

Instead of putting the type flags in the blocker address (pointer),
can't we record the type information outside? It is hard to enforce
the alignment to the locks, because it is embedded in the data
structure. Instead, it is better to record the type as blocker_type
in current task_struct.

Thank you,

> 
> Thanks to Geert for bisecting!
> 
> Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com
> Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> ---
>  include/linux/mutex_types.h | 2 +-
>  include/linux/rwsem.h       | 2 +-
>  include/linux/semaphore.h   | 2 +-
>  3 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mutex_types.h b/include/linux/mutex_types.h
> index fdf7f515fde8..de798bfbc4c7 100644
> --- a/include/linux/mutex_types.h
> +++ b/include/linux/mutex_types.h
> @@ -51,7 +51,7 @@ struct mutex {
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map	dep_map;
>  #endif
> -};
> +} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
>  
>  #else /* !CONFIG_PREEMPT_RT */
>  /*
> diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
> index f1aaf676a874..f6ecf4a4710d 100644
> --- a/include/linux/rwsem.h
> +++ b/include/linux/rwsem.h
> @@ -64,7 +64,7 @@ struct rw_semaphore {
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  	struct lockdep_map	dep_map;
>  #endif
> -};
> +} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
>  
>  #define RWSEM_UNLOCKED_VALUE		0UL
>  #define RWSEM_WRITER_LOCKED		(1UL << 0)
> diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
> index 89706157e622..ac9b9c87bfb7 100644
> --- a/include/linux/semaphore.h
> +++ b/include/linux/semaphore.h
> @@ -20,7 +20,7 @@ struct semaphore {
>  #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
>  	unsigned long		last_holder;
>  #endif
> -};
> +} __aligned(4); /* For hung_task blocker tracking, which encodes type in LSBs */
>  
>  #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
>  #define __LAST_HOLDER_SEMAPHORE_INITIALIZER				\
> -- 
> 2.49.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>