linux-kernel - Re: [PATCH v4] kernel/fork: beware of __put_task

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Message-Id: <20230206170927.9d5afee653dfa0738983dbfa@linux-foundation.org>
Date:   Mon, 6 Feb 2023 17:09:27 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wander Lairson Costa <wander@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Andrei Vagin <avagin@...il.com>,
        linux-kernel@...r.kernel.org (open list),
        Hu Chunyu <chuhu@...hat.com>, Oleg Nesterov <oleg@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v4] kernel/fork: beware of __put_task_struct calling
 context

On Mon,  6 Feb 2023 10:04:47 -0300 Wander Lairson Costa <wander@...hat.com> wrote:

> Under PREEMPT_RT, __put_task_struct() indirectly acquires sleeping
> locks. Therefore, it can't be called from an non-preemptible context.

Well that's regrettable.  Especially if non-preempt kernels don't do
this.

Why does PREEMPT_RT do this and can it be fixed?

If it cannot be fixed then we should have a might_sleep() in
__put_task_struct() for all kernel configurations, along with an
apologetic comment explaining why.