linux-kernel - Re: [PATCH] pidns: remove recursion from free_pid

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Message-ID: <20121005144729.GA20497@redhat.com>
Date:	Fri, 5 Oct 2012 16:47:29 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Vagin <avagin@...nvz.org>
Cc:	linux-kernel@...r.kernel.org, criu@...nvz.org,
	Pavel Emelyanov <xemul@...allels.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] pidns: remove recursion from free_pid_ns

On 10/05, Andrew Vagin wrote:
>
> Here is a stack trace of recursion:
> free_pid_ns(parent)
>   put_pid_ns(parent)
>     kref_put(&ns->kref, free_pid_ns);
>       free_pid_ns
>
> This patch turns recursion into loops.

I think the patch is correct, a couple of minor nits.

> +static inline int __kref_put(struct kref *kref)
> +{
> +	return atomic_sub_and_test(1, &kref->refcount);

perhaps atomic_dec_and_test(&kref->refcount) makes more sense?

> +}
> @@ -138,11 +138,20 @@ void free_pid_ns(struct kref *kref)
>
>  	ns = container_of(kref, struct pid_namespace, kref);
>
> -	parent = ns->parent;
> -	destroy_pid_namespace(ns);
> +	while (1) {
>
> -	if (parent != NULL)
> -		put_pid_ns(parent);
> +		parent = ns->parent;
> +		destroy_pid_namespace(ns);
> +
> +		if (parent == NULL || parent == &init_pid_ns)
                    ^^^^^^^^^^^^^^

Why ns->parent == NULL is only possible if ns == init_pid_ns, right?
But in this case we should not be here. The caller verifies that
initial ns != init_pid_ns, and this loops should stop once we reach
init_pid_ns.

Oleg.

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