linux-kernel - Re: [PATCH] init: Align init_task to avoid conflict with MUTEX

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Date:   Fri, 26 Jun 2020 15:11:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Stafford Horne <shorne@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Marco Elver <elver@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH] init: Align init_task to avoid conflict with MUTEX_FLAGS

On Fri, Jun 26, 2020 at 08:45:22PM +0900, Stafford Horne wrote:

> I think we can make this unconditional.  The only reason I used the condition is
> because the only architecture that sets CONFIG_ARCH_TASK_STRUCT_ON_STACK is ia64
> which already has 64-bit alignment.
> 
> I can change it to (not incorporating max_t(ARCH_MIN_TASKALIGN) as not sure how):

The only arch that has ARCH_MIN_TASKALIGN larger than L1_CACHE_SIZE is
some daft x86 config and it shouldn't be fatal to them.

So with this:

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

> diff --git a/init/init_task.c b/init/init_task.c
> index 15089d15010a..ab6173f8e6a8 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -65,6 +65,7 @@ struct task_struct init_task
>  #ifdef CONFIG_ARCH_TASK_STRUCT_ON_STACK
>         __init_task_data
>  #endif
> +       __aligned(L1_CACHE_BYTES)
>  = {
>  #ifdef CONFIG_THREAD_INFO_IN_TASK
>         .thread_info    = INIT_THREAD_INFO(init_task),
> 
> -Stafford