As a contrasting test, we will now perform the very same thing on our Ubuntu 20.04 LTS VM, which we'll boot via its default generic 'distro' 5.4 Linux kernel that is typically not configured as a 'debug' kernel (here, the CONFIG_DEBUG_ATOMIC_SLEEP kernel config option hasn't been set).
First, we insert our (buggy) driver. Then, when we run our rdwr_drv_secret process in order to write the new secret to the driver, the buggy code path gets executed. However, this time, the kernel does not crash, nor does it report any issues at all (looking at the dmesg(1) output validates this):
$ uname -r
5.4.0-56-generic
$ sudo insmod ./miscdrv_rdwr_spinlock.ko buggy=1
$ ../../ch12/miscdrv_rdwr/rdwr_test_secret w /dev/llkd_miscdrv_rdwr_spinlock "passwdcosts500bucksdude"
Device file /dev/llkd_miscdrv_rdwr_spinlock opened (in write-only mode): fd=3
../../ch12/miscdrv_rdwr/rdwr_test_secret: wrote 24 bytes to...
