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

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Message-ID: <CAM_iQpV+FMvXQDO8o9=x90ybT87OWrSthaxt6soJ_Mhug=vSzA@mail.gmail.com>
Date:   Wed, 8 May 2019 22:34:39 -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 Tue, May 7, 2019 at 7:54 PM Jason Wang <jasowang@...hat.com> wrote:
> This is only true if you can make sure tfile[tun->numqueues] is not
> freed. Either my patch or SOCK_RCU_FREE can solve this, but for
> SOCK_RCU_FREE we need do extra careful audit to make sure it doesn't
> break someting. So synchronize through pointers in tfiles[] which is
> already protected by RCU is much more easier. It can make sure no
> dereference from xmit path after synchornize_net(). And this matches the
> assumptions of the codes after synchronize_net().
>

It is hard to tell which sock_put() matches with this synchronize_net()
given the call path is complicated.

With SOCK_RCU_FREE, no such a problem, all sock_put() will be safe.
So to me SOCK_RCU_FREE is much easier to understand and audit.

Thanks.