Sometimes, a shared resource is a simple integer value. Suppose your driver maintains a shared variable shrd that tells how many device operations are currently outstanding. Normally, even a simple operation such as: sgrd++; would require locking. Some processors might perform that sort of increment in an atomic manner, but you can’t count on it. But a full locking regime seems like overhead for a simple integer value. For cases like this, the kernel provides an atomic integer type called atomic_t, defined in <asm/atomic.h>.
An atomic_t holds an int value on all supported architectures. Because of the way this type works on some processors, however, the full integer range may not be available; thus, you should not count on an atomic_t holding more than 24 bits. The following operations are defined for the type and are guaranteed to be atomic with respect to all processors of an SMP computer. The operations are very fast, because they compile to a single machine instruction whenever possible.
void atomic_set(atomic_t *v, int i);
atomic_t v = ATOMIC_INIT(0);
Set the atomic variable v to the integer value i. You can also initialize atomic values at compile time with the ATOMIC_INIT macro.
