linux-kernel - Re: KCSAN: data-race in exit_signals / prepare

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Message-ID: <20191021134659.GA1339@redhat.com>
Date:   Mon, 21 Oct 2019 15:47:00 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        syzbot <syzbot+492a4acccd8fc75ddfd0@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>, christian@...uner.io,
        deepa.kernel@...il.com, ebiederm@...ssion.com, guro@...com,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com, Will Deacon <will@...nel.org>
Subject: Re: KCSAN: data-race in exit_signals / prepare_signal

On 10/21, Marco Elver wrote:
>
> On Mon, 21 Oct 2019 at 14:00, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > I think this is WONTFIX.
>
> If taking the spinlock is unnecessary (which AFAIK it probably is) and
> there are no other writers to this flag, you will still need a
> WRITE_ONCE(tsk->flags, tsk->flags | PF_EXITING) to avoid the
> data-race.

Or even WRITE_ONCE(tsk->flags, READ_ONCE(tsk->flags) | PF_EXITING) in
theory. But in practice, I do not think compiler can turn

	curent->flags |= PF_EXITING;

into something which temporary clears another flag, say, PF_KTHREAD.

> However, if it is possible that there are concurrent writers setting
> other bits in flags,

No, only current taks should change its ->flags.

Oleg.