linux-kernel - Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if global init task exit

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Message-ID: <20191218080832.GA8408@cqw-OptiPlex-7050>
Date:   Wed, 18 Dec 2019 16:08:32 +0800
From:   chenqiwu <qiwuchen55@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     christian.brauner@...ntu.com, peterz@...radead.org,
        mingo@...nel.org, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if
 global init task exit

On Tue, Dec 17, 2019 at 04:23:33PM +0100, Oleg Nesterov wrote:
> On 12/17, chenqiwu wrote:
> >
> > But in fact, I think atomic_read()
> > can avoid the racy even if both threads exit in parallel, since it is
> > an atomic operation forever.
> 
> Hmm, not sure I understand. atomic_read() is just READ_ONCE(), it can't be
> re-ordered but that is all.
> 
> How can it avoid the race if it is called before atomic_dec_and_test() ?
> 
> Again, suppose that we have 2 exiting threads and signal->live == 2. With
> your patch each thread does atomic_read() before atomic_dec_and_test(),
> both threads can observe atomic_read(signal->live) == 2 simply because
> the counter was not decremented yet.
> 
> Oleg.
>

Yeah, I agree your idea.
Now, if there are no further questions, I will resend this as a proper patch for v3.