netdev - Re: [PATCH] usbnet: fix kernel crash after disconnect

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Date:   Tue, 30 Apr 2019 14:08:01 +0000
From:   Kloetzke Jan <Jan.Kloetzke@...h.de>
To:     "jan@...etzke.net" <jan@...etzke.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "oneukum@...e.com" <oneukum@...e.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>
Subject: Re: [PATCH] usbnet: fix kernel crash after disconnect

Am Montag, den 29.04.2019, 20:48 +0200 schrieb Oliver Neukum:
> On Fr, 2019-04-19 at 09:17 +0200, Jan Klötzke  wrote:
> > Hi David,
> > 
> > On Thu, Apr 18, 2019 at 04:35:44PM -0700, David Miller wrote:
> > > From: Kloetzke Jan <Jan.Kloetzke@...h.de>
> > > Date: Thu, 18 Apr 2019 08:02:59 +0000
> > > 
> > > > I think this assumption is not correct. As far as I understand the
> > > > networking code it is still possible that the ndo_start_xmit callback
> > > > is called while ndo_stop is running and even after ndo_stop has
> > > > returned. You can only be sure after unregister_netdev() has returned.
> > > > Maybe some networking folks can comment on that.
> > > 
> > > The kernel loops over the devices being unregistered, and first it clears
> > > the __LINK_STATE_START on all of them, then it invokes ->ndo_stop() on
> > > all of them.
> > > 
> > > __LINK_STATE_START controls what netif_running() returns.
> > > 
> > > All calls to ->ndo_start_xmit() are guarded by netif_running() checks.
> > > 
> > > So when ndo_stop is invoked you should get no more ndo_start_xmit
> > > invocations on that device.  Otherwise how could you shut down DMA
> > > resources and turn off the TX engine properly?
> > 
> > But you could still race with another CPU that is past the
> > netif_running() check, can you? So the driver has to make sure that it
> > gracefully handles concurrent ->ndo_start_xmit() and ->ndo_stop() calls.
> 
> Looking at dev_direct_xmit(struct sk_buff *skb, u16 queue_id)
> this indeed seems possible. But the documentation says that it is not.
> 
> Dave?
> 
> > Or are there any locks/barriers involved that make sure all
> > ->ndo_start_xmit() calls have returned before invoking ->ndo_stop()?
> 
> Jan,
> 
> could you make versio of your patch that gives a WARNing if this race
> triggers?

Sure. I'll send v2 shortly.

Regards,
Jan