Re: [PATCH] atomic: Specify natural alignment for atomic_t

[Lance Yang]

Hi Finn,

Nice work, thanks for your patch!

On 2025/8/25 10:03, Finn Thain wrote:
> Some recent commits incorrectly assumed the natural alignment of locks.
> That assumption fails on Linux/m68k (and, interestingly, would have failed
> on Linux/cris also). This leads to spurious warnings from the hang check
> code. Fix this bug by adding the necessary 'aligned' attribute.
> 
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Eero Tamminen <oak@xxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Eero Tamminen <oak@xxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@xxxxxxxxxxxxxx/
> Fixes: e711faaafbe5 ("hung_task: replace blocker_mutex with encoded blocker")
> Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
> ---
> I tested this on m68k using GCC and it fixed the problem for me. AFAIK,
> the other architectures naturally align ints already so I'm expecting to
> see no effect there.

Yeah, it is the correct approach for the spurious warnings on architectures
like m68k, where the natural alignment of types can be less than 4 bytes.

> ---
>   include/linux/types.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/types.h b/include/linux/types.h
> index 6dfdb8e8e4c3..cd5b2b0f4b02 100644
> --- a/include/linux/types.h
> +++ b/include/linux/types.h
> @@ -179,7 +179,7 @@ typedef phys_addr_t resource_size_t;
>   typedef unsigned long irq_hw_number_t;
>   
>   typedef struct {
> -	int counter;
> +	int counter __aligned(sizeof(int));
>   } atomic_t;
>   
>   #define ATOMIC_INIT(i) { (i) }

However, as we've seen from the kernel test robot's report on mt6660_chip,
this won't solve the cases where a lock is forced to be unaligned by
#pragma pack(1). That will still trigger warnings, IIUC.

Perhaps we should also apply the follwoing?

diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
index 34e615c76ca5..940f8f3558f6 100644
--- a/include/linux/hung_task.h
+++ b/include/linux/hung_task.h
@@ -45,7 +45,7 @@ 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 +53,6 @@ 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);
 }

Let the feature gracefully do nothing on that ;)