If you have a data structure which is only ever accessed from user context, then you can use a simple semaphore (linux/asm/semaphore.h) to protect it. This is the most trivial case: you initialize the semaphore to the number of resources available (usually 1), and call down_interruptible() to grab the semaphore, and up() to release it. There is also a down(), which should be avoided, because it will not return if a signal is received.

Example: linux/net/core/netfilter.c allows registration of new setsockopt() and getsockopt() calls, with nf_register_sockopt(). Registration and de-registration are only done on module load and unload (and boot time, where there is no concurrency), and the list of registrations is only consulted for an unknown setsockopt() or getsockopt() system call. The nf_sockopt_mutex is perfect to protect this, especially since the setsockopt and getsockopt calls may well sleep.