linux-kernel - Re: [PATCH v9] kernel/fork: beware of __put_task

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Message-Id: <20230516140555.b150a43517d85b7cad75e7da@linux-foundation.org>
Date:   Tue, 16 May 2023 14:05:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Brian Cain <bcain@...cinc.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Stafford Horne <shorne@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrei Vagin <avagin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Yang Shi <shy828301@...il.com>,
        open list <linux-kernel@...r.kernel.org>,
        Hu Chunyu <chuhu@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luis Goncalves <lgoncalv@...hat.com>
Subject: Re: [PATCH v9] kernel/fork: beware of __put_task_struct calling
 context

On Tue, 16 May 2023 20:24:04 +0100 Matthew Wilcox <willy@...radead.org> wrote:

> On Tue, May 16, 2023 at 04:14:41PM -0300, Wander Lairson Costa wrote:
> > +void __put_task_struct_rcu_cb(struct rcu_head *rhp)
> > +{
> > +	struct task_struct *task = container_of(rhp, struct task_struct, rcu);
> > +
> > +	__put_task_struct(task);
> > +}
> > +EXPORT_SYMBOL_GPL(__put_task_struct_rcu_cb);
> 
> Why does this need to be exported when its only caller is within the
> main kernel and cannot possibly be built as a module?

It's referenced by inlined put_task_struct(), which is called from all
over.

However I believe the above definition could be inside #ifdef
CONFIG_PREEMPT_RT, to save a scrap of resources?