Properly drain callouts in the IPFW subsystem

[• hselasky]

Summary

When unloading the dummynet module, there is a slight chance that the dummynet timer might be restarted after the callout is drained. Add a drain variable and use an existing lock to ensure the dymmynet timer is not restarted after the teardown sequence is started.

Callout drain can sleep and must not be called locked.

Test Plan

#!/bin/sh
while true
do
kldunload dummynet
kldunload ipfw
kldload ipfw
kldload dummynet
done

[hiren]

I am not 100% sure if this is the correct approach but its better than whats going on right now. Please go ahead with this patch.
Rasool Al-Saadi also confirmed that this works and avoids panic.

[franco_opnsense.org]

I just hit this and noticed it was never MFC'd. Can this be pushed to STABLE?

[• hselasky]

Feel free to MFC if the patches apply to 10-stable. I won't be able to MFC until end of next week.

[franco_opnsense.org]

Ok, I'll open a bug report for the MFC.

FWIW, applies cleanly to 10.2-RELENG and kldunload works as expected, thanks for this patch!