netdev - Re: [PATCH net V2] tuntap: synchronize through tfiles array instead of tun->numqueues

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Date:   Mon, 6 May 2019 21:54:59 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        YueHaibing <yuehaibing@...wei.com>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net V2] tuntap: synchronize through tfiles array instead
 of tun->numqueues

On Mon, May 6, 2019 at 9:03 PM Jason Wang <jasowang@...hat.com> wrote:
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e9ca1c0..32a0b23 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -700,6 +700,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>                                    tun->tfiles[tun->numqueues - 1]);
>                 ntfile = rtnl_dereference(tun->tfiles[index]);
>                 ntfile->queue_index = index;
> +               rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
> +                                  NULL);
>

How does this work? Existing readers could still read this
tun->tfiles[tun->numqueues - 1] before you NULL it. And,
_if_ the following sock_put() is the one frees it, you still miss
a RCU grace period.

                if (clean) {
                        RCU_INIT_POINTER(tfile->tun, NULL);
                        sock_put(&tfile->sk);


Thanks.