Another technique for avoiding locking which is used fairly widely is to duplicate information for each CPU. For example, if you wanted to keep a count of a common condition, you could use a spin lock and a single counter. Nice and simple.
If that was too slow (it's usually not, but if you've got a
really big machine to test on and can show that it is), you
could instead use a counter for each CPU, then none of them need
an exclusive lock. See DEFINE_PER_CPU(),
get_cpu_var() and
put_cpu_var()
(include/linux/percpu.h).
Of particular use for simple per-cpu counters is the
local_t type, and the
cpu_local_inc() and related functions,
which are more efficient than simple code on some architectures
(include/asm/local.h).
Note that there is no simple, reliable way of getting an exact value of such a counter, without introducing more locks. This is not a problem for some uses.