netdev - Re: [PATCH 6/6] tilegx network driver: initial support

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Message-Id: <201204291115.16127.arnd@arndb.de>
Date:	Sun, 29 Apr 2012 11:15:15 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 6/6] tilegx network driver: initial support

On Saturday 28 April 2012, Chris Metcalf wrote:
> On 4/13/2012 6:34 AM, Arnd Bergmann wrote:
> > On Thursday 12 April 2012, Chris Metcalf wrote:
> >> On 4/10/2012 6:42 AM, Arnd Bergmann wrote:
> >>> Ok, but please remove tile_net_devs then.
> >>> I think a better abstraction for tile_net_devs_for_channel would be
> >>> some interface that lets you add private data to a channel so when
> >>> you get data from a channel, you can extract that pointer from the driver
> >>> using the channel.
> >> I think what would be clearer is to document how and why we are using this
> >> additional data structure.  We do access via both arrays where it is
> >> efficient to do so, so getting rid of either of them doesn't seem right. 
> 
> In the latest round of changes (to be mailed shortly), we eliminated one of
> the arrays entirely.  We now just have an array of net_device pointers
> indexed by channel, which we need since we get packets from the hardware
> and are only given the channel.  To get the device, we have to look it up
> in the array.
> 
> Since this is now the only array of net_device pointers, I eliminated the
> bychannel*() API I discussed in the previous email, since its use didn't
> seem as compelling any more.
> 
> >> Let's keep the "normal" tile_net_devs[] as is, indexed by devno, and make
> >> the tile_net_devs_for_channel[] more abstracted by using the following code:
> > The tile_net_devs still feels dirty. You basically only
> > use it in tile_net_handle_egress_timer(), but there you don't
> > actually take the mutex that protects addition and removal from
> > the array, so it's racy in case of hotplug.
> 
> We don't free the net_device structures themselves, so it's safe to do a
> lookup in the array and then dereference the net_device pointer even if we
> are doing an "ifconfig down" in another thread.  The only way you could
> imagine the net_device getting structures getting freed was via module
> unload, but it turns out that was pretty broken anyway, so I've just
> removed it altogether in the latest version of the patch.  So once you have
> a net_device pointer, it remains valid.

Ok, sounds all good then.

	Arnd
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