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Subject: Re: [PATCH] mac80211: fix race in ieee80211_register_hw()
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Krishna Chaitanya <chaitanya.mgit@gmail.com>,
        Kalle Valo <kvalo@codeaurora.org>
Cc:     Sumit Garg <sumit.garg@linaro.org>,
        linux-wireless <linux-wireless@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>, kuba@kernel.org,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Matthias-Peter =?ISO-8859-1?Q?Sch=F6pfer?= 
        <matthias.schoepfer@ithinx.io>,
        "Berg Philipp (HAU-EDS)" <Philipp.Berg@liebherr.com>,
        "Weitner Michael (HAU-EDS)" <Michael.Weitner@liebherr.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Loic Poulain <loic.poulain@linaro.org>, stable@vger.kernel.org
Date:   Mon, 06 Apr 2020 16:01:06 +0200
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On Mon, 2020-04-06 at 19:25 +0530, Krishna Chaitanya wrote:
> On Mon, Apr 6, 2020 at 6:57 PM Kalle Valo <kvalo@codeaurora.org> wrote:
> > Sumit Garg <sumit.garg@linaro.org> writes:
> > 
> > > On Mon, 6 Apr 2020 at 18:38, Johannes Berg <johannes@sipsolutions.net> wrote:
> > > > On Mon, 2020-04-06 at 16:04 +0300, Kalle Valo wrote:
> > > > > Johannes Berg <johannes@sipsolutions.net> writes:
> > > > > 
> > > > > > On Mon, 2020-04-06 at 15:52 +0300, Kalle Valo wrote:
> > > > > > > Johannes Berg <johannes@sipsolutions.net> writes:
> > > > > > > 
> > > > > > > > On Mon, 2020-04-06 at 15:44 +0300, Kalle Valo wrote:
> > > > > > > > > >     user-space  ieee80211_register_hw()  RX IRQ
> > > > > > > > > >     +++++++++++++++++++++++++++++++++++++++++++++
> > > > > > > > > >        |                    |             |
> > > > > > > > > >        |<---wlan0---wiphy_register()      |
> > > > > > > > > >        |----start wlan0---->|             |
> > > > > > > > > >        |                    |<---IRQ---(RX packet)
> > > > > > > > > >        |              Kernel crash        |
> > > > > > > > > >        |              due to unallocated  |
> > > > > > > > > >        |              workqueue.          |
> > > > > > > > 
> > > > > > > > [snip]
> > > > > > > > 
> > > > > > > > > I have understood that no frames should be received until mac80211 calls
> > > > > > > > > struct ieee80211_ops::start:
> > > > > > > > > 
> > > > > > > > >  * @start: Called before the first netdevice attached to the hardware
> > > > > > > > >  *         is enabled. This should turn on the hardware and must turn on
> > > > > > > > >  *         frame reception (for possibly enabled monitor interfaces.)
> > > > > > > > 
> > > > > > > > True, but I think he's saying that you can actually add and configure an
> > > > > > > > interface as soon as the wiphy is registered?
> > > > > > > 
> > > > > > > With '<---IRQ---(RX packet)' I assumed wcn36xx is delivering a frame to
> > > > > > > mac80211 using ieee80211_rx(), but of course I'm just guessing here.
> > > > > > 
> > > > > > Yeah, but that could be legitimate?
> > > > > 
> > > > > Ah, I misunderstood then. The way I have understood is that no rx frames
> > > > > should be delivered (= calling ieee80211_rx()_ before start() is called,
> > > > > but if that's not the case please ignore me :)
> > > > 
> > > > No no, that _is_ the case. But I think the "start wlan0" could end up
> > > > calling it?

> I am still confused, without ieee80211_if_add how can the userspace
> bring up the interface?

It can add its own interface. Maybe that won't be 'wlan0' but something
else?

like

iw phy0 interface add wlan0 type station
ip link set wlan0 up

johannes