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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 17 Sep 2020 08:51:45 +0200
Message-ID: <CANn89iJOO9cbOqCNpRK4OrZy-L6P8aJJcPMjs5=RHF=fsjEe2Q@mail.gmail.com>
Subject: Re: [PATCH] net: make netdev_wait_allrefs wake-able
To:     Francesco Ruggeri <fruggeri@arista.com>
Cc:     open list <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Taehee Yoo <ap420073@gmail.com>,
        Andrii Nakryiko <andriin@fb.com>,
        Jiri Pirko <jiri@mellanox.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        David Miller <davem@davemloft.net>
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On Thu, Sep 17, 2020 at 8:33 AM Francesco Ruggeri <fruggeri@arista.com> wrote:
>
> >  static inline void dev_put(struct net_device *dev)
> >  {
> > +       struct task_struct *destroy_task = dev->destroy_task;
> > +
> >         this_cpu_dec(*dev->pcpu_refcnt);
> > +       if (destroy_task)
> > +               wake_up_process(destroy_task);
> >  }
>
> I just realized that this introduces a race, if dev_put drops the last
> reference, an already running netdev_wait_allrefs runs to completion
> and then dev_put tries to wake it up.
> Any suggestions on how to avoid this without resorting to
> locking?
>

Honestly I would not touch dev_put() at all.

Simply change the msleep(250) to something better, with maybe
exponential backoff.