netdev - Re: [PATCH net 2/2] cxgb4: Disable interrupts and napi before unregistering netdev

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Message-ID: <20150327085447.GA20708@hariprasad-pc.asicdesigners.com>
Date:	Fri, 27 Mar 2015 14:24:49 +0530
From:	Hariprasad S <hariprasad@...lsio.com>
To:	Lino Sanfilippo <LinoSanfilippo@....de>
CC:	<netdev@...r.kernel.org>, <davem@...emloft.net>,
	<leedom@...lsio.com>, <anish@...lsio.com>, <nirranjan@...lsio.com>,
	<kumaras@...lsio.com>, <swise@...ngridcomputing.com>
Subject: Re: [PATCH net 2/2] cxgb4: Disable interrupts and napi before
 unregistering netdev

On Thu, Mar 26, 2015 at 22:02:14 +0100, Lino Sanfilippo wrote:
> Hi,
> 
> On 26.03.2015 05:34, Hariprasad Shenai wrote:
> > Disable interrupts and quiesce rx before unregistering net device to avoid crash
> > while unloading driver when traffic is flowing through.
> > 
> > Based on original work by Shameem Khalid <shameem@...lsio.com>
> > 
> 
> just out of curiousity: How can an unregister of a netdevice lead to a
> crash caused by traffic? I understand that _freeing_ the device or other
> resources is a bad idea as long as irqs are still active. But unregister
> should not do anything critical (i am asking since I want to avoid such
> problems in my own drivers).
> 
> Regards,
> Lino
> 

When device is unregistered ip_ptr of net_device structure is set to NULL.
Since NAPI is not disabled, every packet goes through ip_error() check.
And in ip_error() function when in_device structure is obtained from netdevice
structure, it ends up in NULL pointer dereference since ip_ptr is already NULL.

Regards,
Hari
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