linux-kernel - Re: [PATCH 1/2] sched: lockless wake-queues

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Message-ID: <20150420182451.6602.qmail@ns.horizon.com>
Date:	20 Apr 2015 14:24:51 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	dave@...olabs.net
Cc:	linux@...izon.com, linux-kernel@...r.kernel.org,
	peterz@...radead.org
Subject: Re: [PATCH 1/2] sched: lockless wake-queues

+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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