Re: [PATCH] init: Align init_task to avoid conflict with MUTEX_FLAGS

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 26, 2020 at 05:44:09AM +0900, Stafford Horne wrote:
> When booting on 32-bit machines (seen on OpenRISC) I saw this warning
> with CONFIG_DEBUG_MUTEXES turned on.

> I traced this to kernel/locking/mutex.c storing 3 bits of MUTEX_FLAGS in
> the task_struct pointer (mutex.owner).  There is a comment saying that
> task_structs are always aligned to L1_CACHE_BYTES.  This is not true for
> the init_task.
> 
> On 64-bit machines this is not a problem because symbol addresses are
> naturally aligned to 64-bits providing 3 bits for MUTEX_FLAGS.  Howerver,
> for 32-bit machines the symbol address only has 2 bits available.
> 
> Fix this by setting init_task alignment to at least L1_CACHE_BYTES.

Whoopsie, sorry about that.

> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Stafford Horne <shorne@gmail.com>
> ---
>  init/init_task.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/init/init_task.c b/init/init_task.c
> index 15089d15010a..d2d2af018d0d 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -64,6 +64,8 @@ unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)]
>  struct task_struct init_task
>  #ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
>  	__init_task_data
> +#else
> +	__aligned(L1_CACHE_BYTES)
>  #endif

Why make this conditional? task_struct_cachep (in kernel/fork.c) has
max_t(int, L1_CACHE_BYTES, ARCH_MIN_TASKALIGN) alignment, so this really
should be aligned on L1_CACHE_BYTES at least.