+struct wake_q_head {
+ struct wake_q_node *first;
+ struct wake_q_node *last;
+};
+
+#define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
+
+#define WAKE_Q(name) \
+ struct wake_q_head name = { WAKE_Q_TAIL, WAKE_Q_TAIL }
Is there some reason you don't use the simpler singly-linked list
construction with the tail being a pointer to a pointer:
struct wake_q_head {
struct wake_q_node *first, **lastp;
};
#define WAKE_Q(name) \
struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
That removes a conditional from wake_q_add:
+/*
+ * Queue a task for later wake-up by wake_up_q(). If the task is already
+ * queued by someone else, leave it to them to deliver the wakeup.
+ *
+ * This property makes it impossible to guarantee the order of wakeups,
+ * but for efficiency we try to deliver wakeups in the order tasks
+ * are added. If we didn't mind reversing the order, a LIFO stack
+ * would be simpler.
+ */
+void wake_q_add(struct wake_q_head *head, struct task_struct *task)
+{
+ struct wake_q_node *node = &task->wake_q;
+
+ /*
+ * Atomically grab the task, if ->wake_q is !nil already it means
+ * its already queued (either by us or someone else) and will get the
+ * wakeup due to that.
+ *
+ * This cmpxchg() implies a full barrier, which pairs with the write
+ * barrier implied by the wakeup in wake_up_list().
+ */
+ if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
+ return;
+
+ get_task_struct(task);
+
+ /*
+ * The head is context local, there can be no concurrency.
+ */
+ *head->lastp = node;
+ head->lastp = &node->next;
+}
It may also be worth commenting the fact that wake_up_q() leaves the
struct wake_q_head in a corrupt state, so don't try to do it again.
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