Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 21 Oct 2019 15:47:00 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Marco Elver <elver@google.com>
Cc:     Christian Brauner <christian.brauner@ubuntu.com>,
        syzbot <syzbot+492a4acccd8fc75ddfd0@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>, christian@brauner.io,
        deepa.kernel@gmail.com, ebiederm@xmission.com, guro@fb.com,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs@googlegroups.com, Will Deacon <will@kernel.org>
Subject: Re: KCSAN: data-race in exit_signals / prepare_signal
Message-ID: <20191021134659.GA1339@redhat.com>
References: <0000000000003b1e8005956939f1@google.com>
 <20191021111920.frmc3njkha4c3a72@wittgenstein>
 <20191021120029.GA24935@redhat.com>
 <CANpmjNMfCK99DoUuR2qRBTLLhrGsYVcpKdtXW7S559tNJ-MO7A@mail.gmail.com>
MIME-Version: 1.0
In-Reply-To: <CANpmjNMfCK99DoUuR2qRBTLLhrGsYVcpKdtXW7S559tNJ-MO7A@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Content-Type: text/plain; charset=WINDOWS-1252
Content-Transfer-Encoding: quoted-printable
Content-Disposition: inline
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 10/21, Marco Elver wrote:
>
> On Mon, 21 Oct 2019 at 14:00, Oleg Nesterov <oleg@redhat.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

=09curent->flags |=3D 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.