Stuck when attaching kretprobe on _raw_write_lock

I was experimenting with bcc on my ubuntu 18.04 machine. The machine is getting stuck when I execute the following code

`from bcc import BPF`
`BPF(text='int kretprobe___raw_spin_lock(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }').trace_print()`

Once above script starts executing the machine is stuck. I don't know if I am doing anything wrong.

But there is no problem when I run the following code. I just changed kretprobe to kprobe.

`from bcc import BPF`
`BPF(text='int kprobe___raw_spin_lock(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }').trace_print()`

The reported entry points are BPF(text=...) and the kretprobe___raw_spin_lock and kprobe___raw_spin_lock handlers; begin by reproducing the difference on Ubuntu 18.04 and tracing the kretprobe attachment path. Compare the two probe types and determine why the kretprobe causes the machine to lock up. Done requires a confirmed cause and a safe correction or documented limitation.

Written by the indexing model from the issue text.