linux-kernel - Re: [PATCH v2] tcp: fix connection reset due to tw hashdance race.

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Message-ID: <CANn89i+JBhj+g564rfVd9gK7OH48v3N+Ln0vAgJehM5xJh32-g@mail.gmail.com>
Date:   Thu, 8 Jun 2023 06:13:04 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     "Duan,Muquan" <duanmuquan@...du.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] tcp: fix connection reset due to tw hashdance race.

On Thu, Jun 8, 2023 at 5:59 AM Duan,Muquan <duanmuquan@...du.com> wrote:
>
> Hi, Eric,
>
> Thanks a lot for your explanation!
>
> Even if we add reader lock,  if set the refcnt outside spin_lock()/spin_unlock(), during the interval between spin_unlock() and refcnt_set(),  other cpus will see the tw sock with refcont 0, and validation for refcnt will fail.
>
> A suggestion, before the tw sock is added into ehash table, it has been already used by tw timer and bhash chain, we can firstly add refcnt to 2 before adding two to ehash table,. or add the refcnt one by one for timer, bhash and ehash. This  can avoid the refcont validation failure on other cpus.
>
> This can reduce the frequency of the connection reset issue from 20 min to 180 min for our product,  We may wait quite a long time before the best solution is ready, if this obvious defect is fixed, userland applications can benefit from it.
>
> Looking forward to your opinions!

Again, my opinion is that we need a proper fix, not work arounds.

I will work on this a bit later.

In the meantime you can apply locally your patch if you feel this is
what you want.