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Date:   Mon, 22 Mar 2021 17:37:05 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     qianli zhao <zhaoqianligood@gmail.com>
Cc:     christian@brauner.io, axboe@kernel.dk,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Collingbourne <pcc@google.com>,
        linux-kernel@vger.kernel.org, Qianli Zhao <zhaoqianli@xiaomi.com>
Subject: Re: [PATCH V3] exit: trigger panic when global init has exited
Message-ID: <20210322163705.GD20390@redhat.com>
References: <1615985460-112867-1-git-send-email-zhaoqianligood@gmail.com>
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 <20210318180450.GA9977@redhat.com>
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 <20210319163225.GB19971@redhat.com>
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Hi,

It seems that we don't understand each other.

If we move atomic_dec_and_test(signal->live) and do

	if (group_dead && is_global_init)
		panic(...);


before setting PF_EXITING like your patch does, then zap_pid_ns_processes()
simply won't be called.

Because:

On 03/21, qianli zhao wrote:
>
> Hi,Oleg
>
> > How? Perhaps I missed something again, but I don't think this is possible.
>
> > zap_pid_ns_processes() simply won't be called, find_child_reaper() will
> > see the !PF_EXITING thread which calls panic().
>
> > So I think this should be documented somehow, at least in the changelog.
>
> This problem occurs when both two init threads enter the do_exit,
> One of the init thread is syscall sys_exit_group,and set SIGNAL_GROUP_EXIT
> The other init thread perform ret_to_user()->get_signal() and found
> SIGNAL_GROUP_EXIT is set,then do_group_exit()->do_exit(),since there
> are no alive init threads it finally goes to
> zap_pid_ns_processes()

No, there is at least one alive init thread. If they all have exited, we have
the thread which calls panic() above.

> and BUG().

so we don't need the SIGNAL_GROUP_EXIT check to avoid this BUG().

What have I missed?

Oleg.