netdev - Re: [PATCH net v2] net: Do not hold the reference for the same sk_rx

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Message-ID: <CAM_iQpWzx8j9ZXZquQn_Jj7-w=Cts-bZxL7UdA39_Vu+=SzuAg@mail.gmail.com>
Date:   Sun, 19 Mar 2017 21:09:38 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Kaiwen Xu <kaiwen.xu@...u.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] net: Do not hold the reference for the same sk_rx_dst

On Sat, Mar 18, 2017 at 9:03 PM, Kaiwen Xu <kaiwen.xu@...u.com> wrote:
> On Sat, Mar 18, 2017 at 08:49:43PM -0700, Cong Wang wrote:
>> On Sat, Mar 18, 2017 at 6:48 PM, Kevin Xu <kaiwen.xu@...u.com> wrote:
>> > In some rare cases, inet_sk_rx_dst_set() may be called multiple times
>> > on the same dst, causing double refcounting. Eventually, it
>> > prevents net_device to be destroyed. The bug manifested as
>> >
>> > unregister_netdevice: waiting for lo to become free. Usage count = 1
>> >
>> > in the kernel log, preventing new network namespace creation.
>> >
>> > Signed-off-by: Kevin Xu <kaiwen.xu@...u.com>
>>
>> Don't know why you don't follow the discussion on your v1...
>>
>> It is protected by bh_lock_sock(), so your patch is not needed
>> at all.
>>
>> Read net/ipv4/udp.c:
>>
>> 1762 /* For TCP sockets, sk_rx_dst is protected by socket lock
>> 1763  * For UDP, we use xchg() to guard against concurrent changes.
>> 1764  */
>
> I probably misunderstood. Do you mean v2 patch is actually not needed or
> the whole workaround is not necessary?

Your patch, no matter v1 or v2, is not needed because we use
bh_lock_sock() to serialize inet_sk_rx_dst_set(), unless you find
a case where we miss the bh_lock_sock(), but you don't say it in
your changelog. "some rare cases" is not enough to justify this bug.