On 14/06/2021 10:43, Greg KH wrote:
On Mon, Jun 14, 2021 at 10:32:05AM +0100, Jonathan Davies wrote:
On 12/06/2021 08:01, Greg KH wrote:
On Fri, Jun 11, 2021 at 03:23:39PM +0000, Jonathan Davies wrote:
When the predictable device naming scheme for NICs is not in use, it is
common for there to be udev rules to rename interfaces to names with
prefix "eth".
Since the timing at which USB NICs are discovered is unpredictable, it
can be interfere with udev's attempt to rename another interface to
"eth0" if a freshly discovered USB interface is initially given the name
"eth0".
Hence it is useful to be able to override the default name. A new usbnet
module parameter allows this to be configured.
Signed-off-by: Jonathan Davies <jonathan.davies@xxxxxxxxxxx>
Suggested-by: Prashanth Sreenivasa <prashanth.sreenivasa@xxxxxxxxxxx>
---
drivers/net/usb/usbnet.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index ecf6284..55f6230 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -72,6 +72,13 @@ static int msg_level = -1;
module_param (msg_level, int, 0);
MODULE_PARM_DESC (msg_level, "Override default message level");
+#define DEFAULT_ETH_DEV_NAME "eth%d"
+
+static char *eth_device_name = DEFAULT_ETH_DEV_NAME;
+module_param(eth_device_name, charp, 0644);
+MODULE_PARM_DESC(eth_device_name, "Device name pattern for Ethernet devices"
+ " (default: \"" DEFAULT_ETH_DEV_NAME "\")");
This is not the 1990's, please do not add new module parameters as they
are on a global driver level, and not on a device level.
The initial name is set at probe-time, so the device doesn't exist yet. So I
felt like it was a choice between either changing the hard-coded "eth%d"
string or providing a driver-level module parameter. Is there a better
alternative?
This has always been this way, why is this suddenly an issue? What
changed to cause the way we can name these devices after they have been
found like we have been for the past decade+?
The thing that changed for me was that system-udevd does *not* have the
backoff and retry logic that traditional versions of udev had.
Compare implementations of rename_netif in
https://git.kernel.org/pub/scm/linux/hotplug/udev.git/tree/src/udev-event.c
(traditional udev, which handles collisions) and
https://github.com/systemd/systemd/blob/main/src/udev/udev-event.c
(systemd-udevd, which does not handle collisions).
I think this logic was removed under the assumption that users of
systemd-udevd would also use the predictable device naming scheme,
meaning renames are guaranteed to not collide with devices being probed.
Also changing the way usb network devices are named is up to userspace,
the kernel should not be involved in this. What is wrong with just
renaming it in userspace as you want to today?
Yes, renaming devices is the responsibility of userspace. Normally udev will
rename a device shortly after it is probed. But there's a window during
which it has the name the kernel initially assigns. If there's other
renaming activity happening during that window there's a chance of
collisions.
Userspace solutions include:
1. udev backing off and retrying in the event of a collision; or
2. avoiding ever renaming a device to a name in the "eth%d" namespace.
Picking a different namespace does not cause a lack of collisions to
happen, you could have multiple usb network devices being found at the
same time, right?
So no matter what, 1) has to happen.
Within a namespace, the "%d" in "eth%d" means __dev_alloc_name finds a
name that's not taken. I didn't check the locking but assume that can
only happen serially, in which case two devices probed in parallel would
not mutually collide.
So I don't think it's necessarily true that 1) has to happen.
Solution 1 is ugly and slow. It's much neater to avoid the collisions in the
first place where possible.
This is not being solved by changing the name as you have to do this no
matter what.
And the code and logic in userspace is already there to do this, right?
This is not a new issue, what changed to cause it to show up for you?
As above, the logic's not there if userspace is using systemd-udevd.
Solution 2 arises naturally from use of the predictable device naming
scheme. But when userspace is not using that, solution 2 may not apply.
Again you always have to do 1 no matter what, so might as well just do
it.
Yes, the problem is a result of userspace decisions, but that doesn't mean
the kernel can't help make things easier.
Ideally, if you _can_ do something in userspace, you should, especially
for policy decisions like naming. That is why udev was created 17 years
ago :)
I'm arguing that a bit of flexibility in the kernel can avoid an
undesirable workaround in userspace. But I can respect the principle you
describe.
Thanks,
Jonathan
