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SubjectRe: [PATCH] RFC: fix ethernet device initialization
Jeff Garzik wrote:
>
> People from time to time point out a wart in ethernet initialization:
>

They sure do. You were away at the time, but I had a 94 file,
140k patch late last year which fixed all this. It's
at

http://www.uow.edu.au/~andrewm/linux/netdevice.patch

and the design doc is at

http://www.uow.edu.au/~andrewm/linux/netdevice2.txt

From a quick look, I think the only substantive difference
here is that my `prepare_etherdev()' function allocates
and reserves the device's name (eth0), but prevents it
from being available in netdevice namespace lookups. This
was done because lots of drivers wanted to do:

init_etherdev(); (Replaced with prepare_etherdev())
printk("%s: something", dev->name);

The changes to dev.c and net_init.c were fairly subtle
and took some thinking about - we should revisit them
if you want to go ahead with this.

The patch all worked OK, was back-compatible with unaltered
drivers, and indeed altered all the drivers. But it kind of
got lost. Too big, too late and dev_probe_lock() was there.

Now, Arjan says that this race is causing oopses. This
surprises me, because current kernels have the the dev_probe_lock()
hack which I put in. This fixes the problem for PCI and Cardbus
drivers. The ISA drivers generally use the dev->init() technique
which is not racy. There isn't a lot left over. Arjan? Which driver?

The other reason I'm surprised that it's causing oopses: most
racy drivers do this:

xxx_probe()
{
init_etherdev();
<initialisation - takes 10s of milliseconds and can sleep>
dev->open = xxx_open;
return;
}
So the vastly most probably failure mode if the race occurs
is this: the interface is opened while dev->open is NULL.
This won't oops. Sure, the interface is screwed because
the open() routine hasn't been called, but it should hang
in there. A subsequent close() of the interface *will*
call dev->close, and I guess the driver is likely to get
upset if its close() routine is called without a corresponding
open().

Yes, we can fix this if we want, and kill off dev_probe_lock().
It'll only take a few days. Do we want? If not, we can
extend the dev_probe_lock() thing to cover probes for
other busses. USB, I guess.


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