+ extern cpu_atomic_t _preempt_forbid_cnt;
+ /*
+ * We don't need to protect the counter against other processes.
+ * The reason why is a bit subtle.
+ *
+ * If a process gets here, preempt_forbid_cnt can be either 0,
+ * or != 0. In the latter case, preemption is already disabled
+ * and no concurrency issues can occur.
+ *
+ * In the former case, we could be preempted just after reading the
+ * value 0 from memory, and a concurrent process might, in fact,
+ * bump the value of preempt_forbid_cnt under our nose!
+ *
+ * BUT: if this ever happens, then we won't get another chance to
+ * run until the other process calls proc_permit() to re-enable
+ * preemption. At this point, the value of preempt_forbid_cnt
+ * must be back to 0, and thus what we had originally read from
+ * memory happens to be valid.
+ *
+ * No matter how hard you think about it, and how complicated you
+ * make your scenario, the above holds true as long as
+ * "preempt_forbid_cnt != 0" means that no task switching is
+ * possible.
+ */
+ ++_preempt_forbid_cnt;