TaiJuWu commented on code in PR #10605:
URL: https://github.com/apache/nuttx/pull/10605#discussion_r1327997823
##########
sched/semaphore/spinlock.c:
##########
@@ -384,4 +384,176 @@ void spin_clrbit(FAR volatile cpu_set_t *set, unsigned
int cpu,
}
#endif
+/****************************************************************************
+ * Name: fair_spinlock_list_init
+ *
+ * Description:
+ * Initialize the link linsk of fair spinlock.
+ * If there is any spinlock node, the node will get lock.
+ *
+ * Input Parameters:
+ * lock_list - A reference to the spinlock list.
+ *
+ * Returned Value:
+ * None
+ *
+ * Assumptions:
+ * None.
+ *
+ ****************************************************************************/
+
+void fair_spinlock_list_init(fair_spinlock_list_t *lock_list)
+{
+ lock_list->lock = SP_UNLOCKED;
+ list_initialize(&lock_list->list);
+}
+
+/****************************************************************************
+ * Name: fair_spinlock_init
+ *
+ * Description:
+ * Initialize the fair spinlock. Tje lock state is set as SP_LOCK util
+ * some give lock to holder.
+ *
+ * Input Parameters:
+ * lock - A reference to the spinlock object to lock.
+ *
+ * Returned Value:
+ * None
+ *
+ * Assumptions:
+ * Not running at the interrupt level.
+ *
+ ****************************************************************************/
+
+void fair_spinlock_init(fair_spinlock_t *lock)
+{
+ lock->lock = SP_LOCKED;
+ lock->holder = this_task();
+ list_initialize(&lock->node);
+}
+
+/****************************************************************************
+ * Name: fair_spin_lock
+ *
+ * Description:
+ * Try to lock the spinlock and insert this lock to lock list.
+ * If list is empty, the caller can get the lock.
+ * If list is not empty, the lock will be inserted to this list.
+ *
+ * Input Parameters:
+ * lock_list - A reference to fair lock list and insert this lock to list.
+ * fair_lock - A reference to the spinlock object to lock.
+ *
+ * Returned Value:
+ * None
+ *
+ * Assumptions:
+ * Not running at the interrupt level.
+ *
+ ****************************************************************************/
+
+void fair_spin_lock(FAR fair_spinlock_list_t *lock_list,
+ FAR fair_spinlock_t *fair_lock)
+{
+ FAR struct tcb_s *rtcb = this_task();
+ fair_spinlock_t *entry;
+ fair_spinlock_t *temp;
+
+#ifdef CONFIG_SCHED_INSTRUMENTATION_SPINLOCKS
+ /* Notify that we have the spinlock */
+
+ sched_note_spinlock(this_task(), &fair_lock->lock, NOTE_SPINLOCK_LOCKED);
+#endif
+
+ /* There is no thread wait this lock */
+
+ spin_lock(&lock_list->lock);
+
+ /* There is no node hold this lock */
+
+ if (list_is_empty(&lock_list->list))
+ {
+ fair_lock->lock = SP_UNLOCKED;
+ list_add_head(&lock_list->list, &fair_lock->node);
+ }
+
+ /* There is a task hold this lock */
+
+ else
+ {
+ /* priority based link list */
+
+ list_for_every_entry_safe(&lock_list->list, entry, temp,
+ struct fair_spinlock_s, node)
+ {
+ if (entry->holder->sched_priority > rtcb->sched_priority)
+ {
+ break;
+ }
+ }
+
+ list_add_before(&entry->node, &fair_lock->node);
+ }
+
+ spin_unlock(&lock_list->lock);
+
+ while (up_testset(&fair_lock->lock) == SP_LOCKED)
Review Comment:
Because RTOS is priority-base, I try to designed a policy.
First we have a queue (lock_list) to main threads which want to get this
lock.
If queue is empty, the first one can get this lock and when it finishes task
it will check the queue is empty or not.
If it is empty just exit. If there is a thread in the queue, it will give
lock to next one in the queue.
And the queue I hope it's priority base so
[here](https://github.com/TaiJuWu/nuttx/blob/fc2b66b139ba0f67706abc7ba88e47c78e692e12/sched/semaphore/spinlock.c#L487)
check the priority and put it in appropriate position.
But lock_list spin lock can let many thread spin so I try to use lock-free
skill to avoid this problem.
What do you think about this idea?
Maybe it's called MCS lock in Linux?
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