netdev - Re: [PATCH bpf-next 1/4] xsk: avoid store-tearing when assigning queues

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Message-ID: <CAJ+HfNiYtnyfcGvAw0X+gNPhpqV8EpCT0Mo=tGX9Oj6XN7NOQA@mail.gmail.com>
Date:   Thu, 22 Aug 2019 15:51:05 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "magnus.karlsson@...el.com" <magnus.karlsson@...el.com>,
        "magnus.karlsson@...il.com" <magnus.karlsson@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>,
        "syzbot+c82697e3043781e08802@...kaller.appspotmail.com" 
        <syzbot+c82697e3043781e08802@...kaller.appspotmail.com>,
        "i.maximets@...sung.com" <i.maximets@...sung.com>
Subject: Re: [PATCH bpf-next 1/4] xsk: avoid store-tearing when assigning queues

On Thu, 22 Aug 2019 at 15:26, Hillf Danton <hdanton@...a.com> wrote:
>
> >
>
> >    /* Make sure queue is ready before it can be seen by others */
>
> >    smp_wmb();
>
>
>
> Hehe, who put mb here and for what?
>

That was from an earlier commit, and it's a barrier paired with the
lock-less reading of queues in xsk_mmap. Uhm... not sure I answered
your question?


Björn




>
>
> >-   *queue = q;
>
> >+  WRITE_ONCE(*queue, q);
>
> >    return 0;
>
>