mdev race conditions

[Bernard Blackham]

Hi,

When I switched from udev+udevd to mdev, booting was much faster and it 
shaved a decent amount from the total image size. However, I've recently 
noticed device nodes sometimes disappearing on boot. The specific 
scenario is:

1. /dev/ttyS1 exists as a device node in the filesystem on boot - prior 
to the actual device existing, and prior to mdev being called.

2. echo /bin/mdev > /proc/sys/kernel/hotplug

3. a module is loaded that provides ttyS1

When this module is loaded, most of the time the existing ttyS1 remains. 
Sometimes however, it is unlinked from the filesystem and is never seen 
again (until mdev -s is run, or next boot, maybe).

I pointed my hotplug to a script to log what mdev was being called as, 
and found that when the module is loaded, there is actually a remove 
event emitted followed by an add event. As there is no locking with 
mdev, sometimes mdev runs the remove after the add and thus the device 
node goes missing.

udev seems to have gotten around this using a daemon that serialises 
events based on the sequence number provided by the kernel.

Is there a lightweight way that mdev can do this or another way to work 
around this problem? I'd also prefer not to have a udevd running 24/7 
unnecessarily, as I imagine most low-memory systems would.

Perhaps something like an on-demand udevd that exists during the flurry 
of uevents from the kernel, waits for things to settle, and then 
processes things in order and exits?

I can't think of a clean way to do this reliably though.

Regards,
Bernard.