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Date: Thu, 17 Dec 2015 17:39:20 -0800
From: Calvin Owens <calvinowens@fb.com>
To: Stephen Hemminger <stephen@networkplumber.org>
CC: <davem@davemloft.net>, <shm@cumulusnetworks.com>,
        <izumi.taku@jp.fujitsu.com>, <linville@tuxdriver.com>,
        <dsa@cumulusnetworks.com>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>
Subject: Re: [PATCH] netconsole: Initialize after all core networking drivers
Message-ID: <20151218013906.GA1715233@devbig337.prn1.facebook.com>
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 <20151217171057.6c0d3b35@xeon-e3>
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On Thursday 12/17 at 17:10 -0800, Stephen Hemminger wrote:
> On Thu, 17 Dec 2015 15:52:39 -0800
> Calvin Owens <calvinowens@fb.com> wrote:
> 
> > With built-in netconsole and IXGBE, configuring netconsole via the kernel
> > cmdline results in the following panic at boot:
> > 
> >     netpoll: netconsole: device eth0 not up yet, forcing it
> >     usb 2-1: new high-speed USB device number 2 using ehci-pci
> >     ixgbe 0000:03:00.0: registered PHC device on eth0
> >     BUG: unable to handle kernel NULL pointer dereference at 0000000000000810
> >     <snip>
> >     Call Trace:
> >      [<ffffffff81578661>] ? vxlan_get_rx_port+0x41/0xa0
> >      [<ffffffff81586828>] ixgbe_open+0x4e8/0x540
> >      [<ffffffff8168045c>] __dev_open+0xac/0x120
> >      [<ffffffff81680506>] dev_open+0x36/0x70
> >      [<ffffffff8169abec>] netpoll_setup+0x23c/0x300
> >      [<ffffffff8169a66a>] ? netpoll_parse_options+0x19a/0x200
> >      [<ffffffff81d797a8>] ? option_setup+0x1f/0x1f
> >      [<ffffffff81d79882>] init_netconsole+0xda/0x262
> >      [<ffffffff81d797a8>] ? option_setup+0x1f/0x1f
> >      [<ffffffff810003a8>] do_one_initcall+0x88/0x1b0
> >      [<ffffffff81d31144>] kernel_init_freeable+0x14a/0x1e3
> >      [<ffffffff81d308f1>] ? do_early_param+0x8c/0x8c
> >      [<ffffffff81778610>] ? rest_init+0x80/0x80
> >      [<ffffffff8177861e>] kernel_init+0xe/0xe0
> >      [<ffffffff8177dc5f>] ret_from_fork+0x3f/0x70
> >      [<ffffffff81778610>] ? rest_init+0x80/0x80
> > 
> > This happens because IXGBE assumes that vxlan has already been initialized.
> > The cleanest way to fix this is to just initialize netconsole after all the
> > other core networking stuff has completed.
> > 
> > Signed-off-by: Calvin Owens <calvinowens@fb.com>
> 
> Fixing this by changing Makefile order is too fragile.
> You are depending on the fact that Makefile order determines link order
> and that determines initialization order. Down that path demons lie.

Hmm, include/linux/init.h explicitly states that "Ordering inside the
subsections is determined by link order", and has since before the
beginning of the history in Git.

I agree it seems magic/scary, but as Eric says I would imagine this
behavior is relied upon in other places.