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

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Message-ID: <20191217152333.GC23152@redhat.com>
Date:   Tue, 17 Dec 2019 16:23:33 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     chenqiwu <qiwuchen55@...il.com>
Cc:     christian.brauner@...ntu.com, peterz@...radead.org,
        mingo@...nel.org, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org, chenqiwu@...omi.com
Subject: Re: [PATCH v2] kernel/exit: do panic earlier to get coredump if
 global init task exit

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.