netdev - Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG

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Date:	Wed, 17 Aug 2016 09:16:02 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Mack <daniel@...que.org>
Cc:	htejun@...com, daniel@...earbox.net, ast@...com,
	davem@...emloft.net, kafai@...com, fw@...len.de,
	pablo@...filter.org, harald@...hat.com, netdev@...r.kernel.org
Subject: Re: [RFC PATCH 3/5] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH
 commands

On Wed, 2016-08-17 at 16:00 +0200, Daniel Mack wrote:

> +		progp = is_ingress ? &cgrp->bpf_ingress : &cgrp->bpf_egress;
> +
> +		rcu_read_lock();
> +		old_prog = rcu_dereference(*progp);
> +		rcu_assign_pointer(*progp, prog);
> +
> +		if (old_prog)
> +			bpf_prog_put(old_prog);
> +
> +		rcu_read_unlock();


This is a bogus locking strategy.

You do not want to use rcu_read_lock()/rcu_read_unlock() here, but
appropriate writer exclusion (a mutex probably, or a spinlock)

Then use rcu_dereference_protected() instead of rcu_dereference(*progp);