[OpenWrt-Devel] netifd hotplug event race condition for monitor interfaces

[Bruno Randolf]

Hello,

I have a system where I want to reload a service when a monitor
interface appears. The monitor interface is configured in
/etc/config/wireless as:

config wifi-iface 'monitor'
        option device 'radio0'
        option ifname 'moni0'
        option mode 'monitor'

Unfortunately netifd events for monitor interfaces are not reliable at
boot time. On some slower systems I never get them, on a Raspberry Pi 2
I get them about 50% of the time.

I have been able to debug it down to netifd/system-linux.c
handle_hotplug_msg() where system_if_force_external() is either true or
false, depending whether /sys/class/net/moni0/phy80211 already exists or
not. Interestingly it works well when it does not exist yet: then I get
the following log, the interface gets UP, and the hotplug script which
restarts my service is called:

netifd: Interface 'moni' is enabled
netifd: Network device 'moni0' link is up
netifd: Interface 'moni' has link connectivity
netifd: Interface 'moni' is setting up now
netifd: Interface 'moni' is now up

But when the /sys/ file does already exist, and
system_if_force_external() is true, I get the following log and the
interface stays down:

netifd: radio0 (1144): command failed: Device or resource busy (-16)
netifd: radio0 (1144): ifconfig: SIOCSIFHWADDR: Invalid argument
netifd: Network device 'moni0' link is up
netifd: Interface 'moni' has link connectivity

root at Rpi2:~# ubus call network.interface.moni status
{
        "up": false,
        "pending": false,
        "available": false,
        "autostart": true,
        "dynamic": false,
        "proto": "none",
        "device": "moni0",
        "data": {

        }
}

I guess the problem is that monitor interfaces are created almost
instantly, in comparison to AP, STA and Ad-hoc interfaces which need
some time to set up...?

Any ideas how to fix this are appreciated, I want to do it, but I am
stuck now.

Thanks,
bruno
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