netdev - Re: [Patch bpf-next v6 04/12] skmsg: avoid lock_sock() in sk_psock

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Date:   Fri, 26 Mar 2021 22:50:35 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v6 04/12] skmsg: avoid lock_sock() in sk_psock_backlog()

On Thu, Mar 25, 2021 at 7:10 PM John Fastabend <john.fastabend@...il.com> wrote:
> Hi Cong,
>
> I'm trying to understand if the workqueue logic will somehow prevent the
> following,
>
>   CPU0                         CPU1
>
>  work dequeue
>  sk_psock_backlog()
>     ... do backlog
>     ... also maybe sleep
>
>                                schedule_work()
>                                work_dequeue
>                                sk_psock_backlog()
>
>           <----- multiple runners -------->
>
>  work_complete
>
> It seems we could get multiple instances of sk_psock_backlog(), unless
> the max_active is set to 1 in __queue_work() which would push us through
> the WORK_STRUCT_DELAYED state. At least thats my initial read. Before
> it didn't matter because we had the sock_lock to ensure we have only a
> single runner here.
>
> I need to study the workqueue code here to be sure, but I'm thinking
> this might a problem unless we set up the workqueue correctly.
>
> Do you have any extra details on why above can't happen thanks.

Very good question!

I thought a same work callback is never executed concurrently, but
after reading the workqueue code, actually I agree with you on this, that
is, a same work callback can be executed concurrently on different CPU's.

Limiting max_active to 1 is not a solution here, as we still want to keep
different items running concurrently. Therefore, we still need a mutex here,
just to protect this scenario. I will add a psock->work_mutex inside
sk_psock_backlog().

Thanks!