David Lee's questions

the config option which determines whether the udp_tunnel module is compiled into the kernel is CONFIG_NET_UDP_TUNNEL. And the CONFIG_NET_UDP_TUNNEL in .config of Ubuntu is M, which should not be loaded into kernel automatically. But the result below shows that the udp_tunnel is loaded into kernel, which is confused to me.

$ modinfo udp_tunnel
filename:       /lib/modules/5.3.0-51-generic/kernel/net/ipv4/udp_tunnel.ko
license:        GPL
srcversion:     0A315BA6124B0664F4D23FB
depends:        
retpoline:      Y
intree:         Y
name:           udp_tunnel
vermagic:       5.3.0-51-generic SMP mod_unload 
signat:         PKCS#7
signer:         
sig_key:        
sig_hashalgo:   md4

$ echo $?
0

shows the modules udp_tunnel is loaded into the kernel and

I am developing a program which causes deadlocks. And I want Ubuntu to warn about all the deadlocks in the kernel, so I can know if my program succeeds. It's required that kernel can detect deadlocks automatically. And https://stackoverflow.com/questions/20892822/how-to-use-lockdep-feature-in-linux-kernel-for-deadlock-detection says that I should enable CONFIG_LOCKDEP and CONFIG_PROVE_LOCKING in .config. However, CONFIG_LOCKDEP and CONFIG_PROVE_LOCKING both depend on CONFIG_DEBUG_LOCK_ALLOC. When I enable these three options and compile, it gives me this error:

Kernel built with CONFIG_DEBUG_LOCK_ALLOC which is incompatible with the CDDL license and will prevent the module linking stage from succeeding. You must rebuild your kernel without this option enabled.

How should I solve this?

Previously, I thought Ubuntu-4.15.0-76.86 is based on 4.15.18

However, I just found it has some commits from kernel. For example ,ef61eb43ada6c1d6b94668f0f514e4c268093ff3 is a commit from linux 5.0 In ubuntu source code directory, the output of

git show ef61eb43ada6c1d6b94668f0f514e4c268093ff3

is totally the same as in mainline kernel. The result of

git describe ef61eb43ada6c1d6b94668f0f514e4c268093ff3

is also v5.0-12878-gef61eb43ada6. So Ubuntu-4.15.0-76.86 also forks from 5.0 and other kernel versions, not only from 4.15.18? They all exists in the source code directory as a branch?

Thanks

When I do some change to source code of ubuntu kernel, I have to fakeroot debian/rules clean and then compile the ubuntu kernel from scratch. But compiling the whole kernel spends a lot of time. Is there a way that I can only compile changed source code to save time? Thanks