linux-kernel - Re: [patch 4/5] sched: Delay task stack freeing on RT

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Message-ID: <87a6js4gb3.ffs@tglx>
Date:   Fri, 01 Oct 2021 22:54:08 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [patch 4/5] sched: Delay task stack freeing on RT

On Fri, Oct 01 2021 at 12:02, Andy Lutomirski wrote:
> On Fri, Oct 1, 2021 at 11:48 AM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> On Fri, Oct 1, 2021 at 10:24 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> >
>
>> ISTM it would be conceptually for do_exit() to handle its own freeing
>> in its own preemptible context.  Obviously that can't really work,
>> since we can't free a task_struct or a task stack while we're running
>> on it.  But I wonder if we could approximate it by putting this work
>> in a workqueue so that it all runs in a normal schedulable context.
>> To make the shell script case work nicely, we want to release the task
>> stack before notifying anyone waiting for the dying task to exit, but
>> maybe that's doable.  It could involve some nasty exit_signal hackery,
>> though.
>
> I'm making this way more complicated than it needs to be.  How about
> we unaccount the task stack in do_exit and release it for real in
> finish_task_switch()?  Other than accounting, free_thread_stack
> doesn't take any locks.

Right.