netdev - Re: [PATCH net-next 2/5] r8152: replace array with linking list for rx information

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Date:   Wed, 7 Aug 2019 11:21:39 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Hayes Wang <hayeswang@...ltek.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        nic_swsd <nic_swsd@...ltek.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH net-next 2/5] r8152: replace array with linking list for
 rx information

On Wed, 7 Aug 2019 04:34:24 +0000, Hayes Wang wrote:
> Jakub Kicinski [mailto:jakub.kicinski@...ronome.com]
> > >  static int rtl_stop_rx(struct r8152 *tp)
> > >  {
> > > -	int i;
> > > +	struct list_head *cursor, *next, tmp_list;
> > > +	unsigned long flags;
> > > +
> > > +	INIT_LIST_HEAD(&tmp_list);
> > >
> > > -	for (i = 0; i < RTL8152_MAX_RX; i++)
> > > -		usb_kill_urb(tp->rx_info[i].urb);
> > > +	/* The usb_kill_urb() couldn't be used in atomic.
> > > +	 * Therefore, move the list of rx_info to a tmp one.
> > > +	 * Then, list_for_each_safe could be used without
> > > +	 * spin lock.
> > > +	 */  
> > 
> > Would you mind explaining in a little more detail why taking the
> > entries from the list for a brief period of time is safe?  
> 
> Usually, it needs the spin lock before accessing the entry
> of the list "tp->rx_info". However, for some reasons,
> if we want to access the entry without spin lock, we
> cloud move all entries to a local list temporally. Then,
> we could make sure no other one could access the entries
> included in the temporal local list.
> 
> For this case, when I move all entries to a temporal 
> local list, no other one could access them. Therefore,
> I could access the entries included in the temporal local
> list without spin lock.

I see, thanks for the explanation.