linux-kernel - Re: [PATCH 2/8] kernel/fork: Duplicate task

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Message-ID: <Ygo/WWSQHFkGn095@linutronix.de>
Date:   Mon, 14 Feb 2022 12:39:05 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 2/8] kernel/fork: Duplicate task_struct before stack
 allocation.

On 2022-02-11 15:42:38 [-0800], Andy Lutomirski wrote:
> On 1/25/22 07:26, Sebastian Andrzej Siewior wrote:
> > alloc_thread_stack_node() already populates the task_struct::stack
> > member except on IA64. The stack pointer is saved and populated again
> > because IA64 needs it and arch_dup_task_struct() overwrites it.
> 
> I understand the problem, I think.
> 
> > 
> > Allocate thread's stack after task_struct has been duplicated as a
> > preparation.
> > 
> 
> But I don't understand this.  How does this patch relate to the problem?

So I duplicate the task-struct, assign the stack pointer in
alloc_thread_stack_node() with no need to update the stack pointer
later. Otherwise arch_dup_task_struct() would reset the pointer.

> Also, you appear to be missing a change to the free_stack and free_tsk code
> at the end of dup_task_struct().

It looks right. What am I missing?

Sebastian