linux-kernel - Re: [PATCH 1/1] hung_task: fix warnings caused by unaligned lock pointers

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Message-Id: <20250826134948.4f5f5aa74849e7f56f106c83@kernel.org>
Date: Tue, 26 Aug 2025 13:49:48 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Lance Yang <lance.yang@...ux.dev>
Cc: akpm@...ux-foundation.org, fthain@...ux-m68k.org, geert@...ux-m68k.org,
 senozhatsky@...omium.org, amaindex@...look.com, anna.schumaker@...cle.com,
 boqun.feng@...il.com, ioworker0@...il.com, joel.granados@...nel.org,
 jstultz@...gle.com, kent.overstreet@...ux.dev, leonylgao@...cent.com,
 linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
 longman@...hat.com, mingo@...hat.com, mingzhe.yang@...com,
 oak@...sinkinet.fi, peterz@...radead.org, rostedt@...dmis.org,
 tfiga@...omium.org, will@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/1] hung_task: fix warnings caused by unaligned lock
 pointers

On Sat, 23 Aug 2025 13:00:36 +0800
Lance Yang <lance.yang@...ux.dev> wrote:

> From: Lance Yang <lance.yang@...ux.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, the runtime checks are adjusted. The first WARN_ON_ONCE in
> hung_task_set_blocker() is changed to a simple 'if' that returns silently
> for unaligned pointers. The second, now-invalid WARN_ON_ONCE in
> hung_task_clear_blocker() is then removed.
> 
> Thanks to Geert for bisecting!
> 
> Reported-by: Geert Uytterhoeven <geert@...ux-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@...r.kernel.org>
> Signed-off-by: Lance Yang <lance.yang@...ux.dev>

Looks good to me. I think we can just ignore it for
this debugging option.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> ---
>  include/linux/hung_task.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
> index 34e615c76ca5..69640f266a69 100644
> --- a/include/linux/hung_task.h
> +++ b/include/linux/hung_task.h
> @@ -20,6 +20,10 @@
>   * always zero. So we can use these bits to encode the specific blocking
>   * type.
>   *
> + * Note that on architectures like m68k with only 2-byte alignment, the
> + * blocker tracking mechanism gracefully does nothing for any lock that is
> + * not 4-byte aligned.
> + *
>   * Type encoding:
>   * 00 - Blocked on mutex			(BLOCKER_TYPE_MUTEX)
>   * 01 - Blocked on semaphore			(BLOCKER_TYPE_SEM)
> @@ -45,7 +49,7 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
>  	 * If the lock pointer matches the BLOCKER_TYPE_MASK, return
>  	 * without writing anything.
>  	 */
> -	if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK))
> +	if (lock_ptr & BLOCKER_TYPE_MASK)
>  		return;
>  
>  	WRITE_ONCE(current->blocker, lock_ptr | type);
> @@ -53,8 +57,6 @@ static inline void hung_task_set_blocker(void *lock, unsigned long type)
>  
>  static inline void hung_task_clear_blocker(void)
>  {
> -	WARN_ON_ONCE(!READ_ONCE(current->blocker));
> -
>  	WRITE_ONCE(current->blocker, 0UL);
>  }
>  
> -- 
> 2.49.0
> 


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