+/**
+ * Disable preemptive task switching.
+ *
+ * The scheduler maintains a global nesting counter. Task switching is
+ * effectively re-enabled only when the number of calls to proc_permit()
+ * matches the number of calls to proc_forbid().
+ *
+ * \note Calling functions that could sleep while task switching is disabled
+ * is dangerous and unsupported.
+ *
+ * \note calling proc_forbid() from within an interrupt is illegal and
+ * meaningless.
+ *
+ * \note proc_permit() expands inline to 1-2 asm instructions, so it's a
+ * very efficient locking primitive in simple but performance-critical
+ * situations. In all other cases, semaphores offer a more flexible and
+ * fine-grained locking primitive.
+ *
+ * \sa proc_permit()
+ */
+INLINE void proc_forbid(void)
+{
+ #if CONFIG_KERN_PREEMPT
+ 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;
+
+ /*
+ * Make sure _preempt_forbid_cnt is flushed to memory so the
+ * preemption softirq will see the correct value from now on.
+ */
+ MEMORY_BARRIER;
+ #endif
+}
+
+/**
+ * Re-enable preemptive task switching.
+ *
+ * \sa proc_forbid()
+ */
+INLINE void proc_permit(void)
+{
+ #if CONFIG_KERN_PREEMPT
+
+ /*
+ * This is to ensure any global state changed by the process gets
+ * flushed to memory before task switching is re-enabled.
+ */
+ MEMORY_BARRIER;
+ extern cpu_atomic_t _preempt_forbid_cnt;
+ /* No need to protect against interrupts here. */
+ ASSERT(_preempt_forbid_cnt != 0);
+ --_preempt_forbid_cnt;
+
+ /*
+ * This ensures _preempt_forbid_cnt is flushed to memory immediately
+ * so the preemption interrupt sees the correct value.
+ */
+ MEMORY_BARRIER;
+
+ #endif
+}
+
+/**
+ * \return true if preemptive task switching is allowed.
+ * \note This accessor is needed because _preempt_forbid_cnt
+ * must be absoultely private.
+ */
+INLINE bool proc_preemptAllowed(void)
+{
+ #if CONFIG_KERN_PREEMPT
+ extern cpu_atomic_t _preempt_forbid_cnt;
+ return (_preempt_forbid_cnt == 0);
+ #else
+ return true;
+ #endif
+}
+
+/** Deprecated, use the proc_preemptAllowed() macro. */
+#define proc_allowed() proc_preemptAllowed()
+