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

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Message-ID: <CALCETrXk0i50vyWwou0AT0jtb7V2pqZiJW_7_dqrthi1N19qeg@mail.gmail.com>
Date:   Fri, 1 Oct 2021 12:02:19 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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 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.

--Andy