We have created a simple device driver code example in the Linux Kernel Programming (Part 2) book in the Writing a Simple misc Character Device Driver chapter; that is, miscdrv_rdwr. There, we wrote a simple misc class character device driver and used a user space utility program (miscdrv_rdwr/rdwr_drv_secret.c) to read and write a (so-called) secret from and to the device driver's memory.
However, what we glaringly (egregiously is the right word here!) failed to do in that code is protect shared (global) writeable data! This will cost us dearly in the real world. I urge you to take some time to think about this: it isn't viable that two (or three or more) user mode processes open the device file of this driver, and then concurrently issue various I/O reads and writes. Here, the global shared writable data (in this particular case, two global integers and the driver context data structure) could easily...
