Manual Pages  — DPCPU

For example, it may be desirable to protect access using critical_section(9) to prevent both preemption and migration during use. Alternatively, it may be desirable to cache the CPU ID at the start of a sequence of accesses, using suitable synchronization to make non-atomic sequences safe in the presence of migration.

DPCPU_DEFINE_STATIC(int, foo_int);
DPCPU_DEFINE_STATIC(struct mutex, foo_lock);

void
foo_int_increment(void)
{
    int cpu, value;

    /* Safe as atomic access. */
    atomic_add_int(DPCPU_PTR(foo_int), 1);

    /*
     * Protect with a critical section, which prevents preemption
     * and migration.  However, access to instances from remote CPUs
     * is not safe, as critical sections prevent concurrent access
     * only from the current CPU.
     */
    critical_enter();
    value = DPCPU_GET(foo_int);
    value++;
    DPCPU_SET(foo_int, value);
    critical_exit();

    /*
     * Protect with a per-CPU mutex, tolerating migration, but
     * potentially accessing the variable from multiple CPUs if
     * migration occurs after reading curcpu.  Remote access to a
     * per-CPU variable is safe as long as the correct mutex is
     * acquired.
     */
    cpu = curcpu;
    mtx_lock(DPCPU_ID_PTR(cpu, foo_lock));
    value = DPCPU_ID_GET(cpu, foo_int);
    value++;
    DPCPU_ID_SET(cpu, foo_int);
    mtx_unlock(DPCPU_ID_PTR(cpu, foo_lock));
}