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Subject: Re: [PATCH] notifiers: double register detection
To:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
References: <04127e71-4782-9bbb-fe5a-7c01e93a99b0@virtuozzo.com>
From:   Vasily Averin <vvs@virtuozzo.com>
Message-ID: <70db0274-2dfc-8cd1-9e0b-74e035a089d6@virtuozzo.com>
Date:   Tue, 12 Mar 2019 11:20:40 +0300
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Dear Andrew,
could you please pick up this patch?

Few years ago I investigated described problem, host was cycled because of notifier's list corruption.
I've prepared this patch applied it to OpenVZ kernel and sent this patch first time, but nobody commented it.
Later it helped us to detect similar problem on OpenVz kernel.

Mistakes with notifiers register can happen for example during subsystem initilazation
from different namespaces, or because of lost unregister in roll-back path on initialization failures.

Thank you,
	Vasily Averin

On 3/12/19 11:06 AM, Vasily Averin wrote:
> By design notifiers can be registerd once only,
> 2nd register attempt called by mistake silently corrupts notifiers list.
> 
> Proposed check cannot prevent described problem,
> however it allows to detect its reason quickly without coredump analyze.
> 
> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
> ---
>  kernel/notifier.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/notifier.c b/kernel/notifier.c
> index 6196af8a8223..bfc95b3e4235 100644
> --- a/kernel/notifier.c
> +++ b/kernel/notifier.c
> @@ -22,6 +22,7 @@ static int notifier_chain_register(struct notifier_block **nl,
>  		struct notifier_block *n)
>  {
>  	while ((*nl) != NULL) {
> +		WARN_ONCE(((*nl) == n), "double register detected");
>  		if (n->priority > (*nl)->priority)
>  			break;
>  		nl = &((*nl)->next);
>